I've been on both sides of this type of thing. I've put together limited partnerships as the general partner to buy racehorses and I've also participated in one as a limited partner. My experience pretty much comes down to this: If you aren't paying a training bill, you aren't really a partner.
Also, this kind of set up works much better with a harness horse. A standardbred can race every week. Thoroughbreds are so fucking fragile. Who even knows if that bastard will race at all. Basically, you're paying the five hundo to say you own a horse and
maybe get into the winner's circle for a picture with "your" horse. If you go to Arlington every day it's probably worth it just to avoid the exorbitant and unconscionable admission fees.
These guys run some of the better partnerships that I'm familiar with:
http://www.vipstable.com/