I don't have stats but from what i have looked at if you start to back a city class winner from a spell say as a 3yo you could make a profit over a lifetime of racing.

The main problem i found was having to many horses in your stable so even when you had winners that made a profit your commitments to other runners left a loss at that time, if you decide to only have a few runners it could be weeks even months before the wins.

I thought about having a max run of outs from a spell maybe 5 that way you don't over commit to a horse that may be 12 races before a win.