how much are offshore gambling winnings taxed?

Placing a wager on sports is totally legal at the federal level in the US and is not governed by the Wire Act. The act of accepting wagers is what is covered by the wire act, an important distinction. 

Let me quote the ruling that has stood since Nov 25, 1981 and has NEVER been challenged (USA vs Robert Baborian 528 F.Supp 324 paragraph 5):

Congress intended the "business of gambling" to mean bookmaking, i.e. the taking and laying off of bets, and not mere betting. The provocative question is whether this is still the proper definition when the bettor wagers substantial sums and displays the sophistication of an expert in his knowledge of odds making. This court concludes that the statute simple does not covers such a situation. This court finds that Congress never intended to include a social bettor within the prohibition of the statute and that Congress did not contemplate prohibiting the activities of mere bettors, even where, as with Mr Baborian, they bet large sums of money with a great deal of sophistication. 

The truth of the matter is that the DOJ does interpret the law the way I stated it, and that is evidenced by the fact that there has not been a single prosecution at the Federal level since the case I cited (USA vs Baborian 1981). Not one single bettor has been brought up on federal charges since 1981 unless there was tax evasion, money laundering, or RICO thrown at them. The Wire Act simply applies ONLY to those in the business of gambling, and as defined by the judicial system that means someone who TAKES bets. The following are some articles on the subject - keep in mind when reading them that when they say sportsbetting is illegal under the wire act - the key definition is that of someone "in the business of gambling" which has been defined to exclude the person placing bets.

https://www.playwinningpoker.com/online/poker/legal/
https://www.gambling-law-us.com/Federal-Laws/wire-act.htm
https://www.unc.edu/courses/2006spring/law/357c/001/projects/dsmatthe/federal.html(important pertinent text quoted in italics below - its about the 4th paragraph in the link)

Criticisms of the Wire Act’s Applicability to Online Gambling

Even if the Wire Act is shown to apply to internet gambling it would only apply to gambling operators, since it expressly states that it applies only to those “engaged in the business of betting or wagering.” 18 U.S.C. 1084, U.S. Code Collection, Cornell Law School , athttps://www.law.cornell.edu/uscode/html/uscode18/usc_sec_18_00001084----000-.html .Obviously, this would exclude individual bettors from any liability under the Act. This point is fairly uncontroversial, and is supported by the language of Congressman Emanuel Celler, then Chairman of the House Judiciary Committee, who stated during debate on the Act that “[t]his bill only gets after the bookmaker, the gambler who makes it his business to take bets or to lay off bets. . . It does not go after the causal gambler.” Jeffrey R. Rodefer, Internet Gambling in Nevada : Overview of Federal Law Affecting Assembly Bill 466, 8-13, athttps://web.archive.org/web/20040303190351/http:/ag.state.nv.us/hottopics/int_gamb_nv.pdf(quoting United States v. Baborian, 528 F. Supp. 324, 328 (D.R.I. 1981) (quoting 107 Cong.Rec. 16,534 (1961))). 

At the state level, certain states have specific laws that make gambling illegal. Even other states (nevada, california, louisians, and washington) have specifically made online gambling illegal. Any way you slice it, NOBODY HAS EVER BEEN PROSECUTED FOR ONLINE GAMBLING. The closest thing was Jeffrey Trauman of North Dakota - who paid a 500 dollar fine and never went to court in the only known attempt at harassing an online gambler in US history.

https://www.gamblingandthelaw.com/columns/161_Busted_Online_Bet.htm

The politicians love to stand up and say that online gambling is illegal because it helps their causes and they can say whatever they want to further their agendas. The proof is simply that it is totally untrue. Sports bettors are exempt because they are not in the "business of gambling".

You might have heard of the UIGEA that was passed under GWB in 2006.  This has nothing to do with the gambler, and everything to do with the credit card processors and banks.  This does not criminalize anything for the player.  In a nutshell, it makes it harder to get money back and forth, but if you can get money back and forth, you have done nothing illegal as the gambler.

If sports gambling was already illegal under the wire act why would they be trying to pass the proposition to update the wire act? They wouldnt need to. Politicians love to say that sports betting is illegal but then at the same time are trying to pass a bill that makes it illegal. Does that make sense to you? That should be proof alone.

Simple google searches on the subject will all be consistent - if it is a politician talking they will say the wire act makes it illegal and if it is an attorney or independent party they will state the facts - that no federal statute covers online gambling (poker or sportsbetting) if you are placing wagers and not in the business of gambling as defined in 1981.

Now this is all at the FEDERAL LEVEL. Every state is different on their state statutes with regards to gambling, but they are generally considered to be soft laws considering the lack of effort to even enforce them. But the federal law is clear to anyone who does not have an agenda, take bets and you are in trouble, place bets and you are ok.

So as seen throughout this entire thread, if someone doesn't agree with you, they're a moron? Is that your favorite 'cop-out' word? I have read nothing but your whim opinion on outdated material that you give reference to, published by yourself in other threads, or a 1981 appellate court decision that has no relation to the discussion in this particular thread, But I am a moron? BTW the U.S. does not set a tax structure of citizens working abroad, THE COUNTRY THEY ARE WORKING IN DOES, according to you everyone working outside the USA would be taxed twice..sure they would,dahhhh But I Am a moron. You justify absolutely nothing with your 'lame' egotistical degrading commentary, But I as others in this who disagree with your assessment are simply 'morons' actually, I see no one in this thread that agree with anything you say, (basically because they have no knowledge or comprehension as to what your even talking about)  other than picking up losing tickets at the Racetrack for tax auditing purposes.. (that's as stupid as it gets) you debate, discuss, or argue the facts, calling names is about as immature as it goes, you sound like a teenager writing an article for the National Enquire....Old socks with new shoes