Great thread!
Just wanted to add that if you are after a specific breed but don't have the $$ for a puppy from a reputable breeder, it's still worth checking the breed clubs and local registered breeders in your state. They will be able to point you in the direction of specific rescue programs (such as the Jack Russel and Greyhound ones that have been mentioned - these organisations exist for most breeds) and/or might have an ex-show dog needing to be rehomed. Much cheaper, but you're doing a good deed and likely getting a dog with fewer health issues. Plus you get to know a reputable breeder and have a new support network with a wealth of breed-specific information