While you are calculating do not calculate yourself out of your rent helper. If you make the price too high your sister will decide to get rid of her horse and you will be paying all the lease yourself.
Easy... I HAVE talked to her , we live in an apartment together and are close. I've asked her if she'd like me to rehome him but she likes him and wants to keep him she just doesn't have an interest in riding anymore (she was never super motivated). She works two jobs and goes to college and it just doesn't work in her life. I know this and I was asking for advice on PRICING not figuring out my sisters life.
Well then I think you have it easy. Lay out the options for rehoming or keeping. Pricing is going to be whatever you want to charge to take care of this horse. Ako gave you a formula, but really, the pricing is completely your call.
Having two sisters, I would never suggest trying to figure out anyone's life. Your sister sounds superbusy and in need of simplification in her life. It's her call on whether she wants to pay to keep the horse around or not. If she rehomes, perhaps she can let go of one of those extra jobs.
Again, nothing is better than a calm adult conversation and letting her decide whatever works for her.