Sadly, some "rescues" are nothing but a scam. I know of one here in Missouri who charges $750, like you said, for Cavaliers. The dogs are donated to them in most cases or they get them from dog auctions but don't pay much. They claim to do heart surgery, leg surgery etc when needed and that is why the dogs are so expensive.
Granted most rescues spay/neuter their dogs so if they don't have a low cost clinic to access it may run $150. Shots are about $25 for boosters and rabies, so they may have about $200-$300 tied up in the dog, but asking $750 Plus shipping is about $300 for air.
One of the biggest scammers around is the HSUS. They don't own, support or maintain ANY shelters yet people are duped every day into donating big bucks for their "efforts to rescue these helpless animals" I wish our state and federal prosecutors would look into that scam!
Don't give up. Look at nextdaypets.com, petfinder.com and some of the other sites and you will find a great dog at a reasonable price.