50% of the winnings.
If your horse finished with a $10k payout, then the staker is out $5k.
On a side note,
I have a much more generous offer...
If you pay $10k I will put out the rake, and Will pay you back 80% of my winnings.
Come on, Where are you going to find a better deal than that?
I'd move this to the spam section but I don't think there will be many replies.