Well, I have the EX500, and the EX510 seems to be very similar, if not the same except it's got different housings and cables - at least they both seem to have the same 13.5mm dynamic driver.
Shame you can't afford the EX600 (specially if you're based in the USA where they're a lot cheaper than in other countries) because you just don't know just how good they are - the EX600 price is definitely misleading, but I also know that $125-$140 is indeed a lot of money for a lot of people. If I were in your position, knowing what I know now, I'd try to wait and save a little and try and get them. Only a year ago there was no IEM with such sound quality that cost $130, or $200-$250 even. Now, if $100 is already too much to begin with, then the EX510 may prove to be a very nice phone, and for all you know even an improvement on the EX500 (though I honestly doubt it).
Just make sure you get the EX510 from an authorised dealer as there are tons of fake Sony/Sennheiser/Bose IEMs being sold on eBay and even by quite a few Amazon Marketplace sellers (ie not directly from Amazon), though I don't know if there are any fake EX510 yet - there are tons of EX700, EX500 & EX300 fakes out there.