I think things happen alot quicker with dogs! Most are racing now at about 20 months or so, and there isn't that "We'll spell him and see if he grows" type of situation where they might have 1-2 runs in a prep and then disappear to a paddock for months on end, next thing you know they are 4-5 years old and never really blossomed into what was hoped! The pain with Dogs is short and sharp you pretty much know after about 6 or so starts if they are going to make it.
A good racing animal will pay for itself and may even have you in the black, its the slow ones that cost the $!! In my opinion a slow horse costs a hell of a lot more than a slow dog!