Are you taking all the losses if you lose, or just partnering up? Are you splitting the money based on each individual session, or the whole road trip? What about other costs (travel, food, etc.). All these factors make a HUGE differnece.
Something around 50/50 if the backer covers the loss is typical. From there, you can easily go 10% in either direction.
If the backer wants too much more than 60/40, the risk of dumping becomes much higher.
I personally would never back another player. I used to back a top local player at a 50/50 split, where I took the losses, on an individual session basis. After about a year of this and all was said and done, I lost, and my horse won. I realized the horse had to win about 75% of the time for me to just break EVEN.
Your experience may vary of course.