Depending on the city, North American rates for regular club residents seem to fall in the 100 – 500 a night range (as Terry said, it varies from one city to the next). To add to that, there isn’t a lot of rhyme or reason to which cities have higher rates and which have lower rates.
My rule of thumb for travelling is that I need to walk away with at least as much as I would doing a show locally (so travel costs, work permit fees (US requires visas for DJs), plus my usual local fee). Running a quick currency conversion, you should be asking at least $250. And if they are willing to fly you in from London twice a month, I don’t think that is an unreasonable ask at all.
Of course, if you already garner that level of demand, you’re going to be able to up those rates pretty quickly! Cha-ching! 😀