Part-loaning a horse?

Prices vary quite a bit depending on where you live, the situation, the horse in question, etc. There's really no way to give you a price estimate from the info given. Ask around. Go to boarding facilities, ranches, and other horse properties in your area and ask if anyone there is interested in half-leasing their horse to someone. You don't have to lease from a trainer, but many trainers are happy to help you find the right horse to lease if you ask them.

I'm very pro leasing and half leasing, especially for young and/or green riders. It's a great way to get some of the responsibility and freedom of having "your own" horse, but it minimizes the financial responsibility on your end. It also gives you the chance to outgrow the potential of your lease horse in your training. You should be constantly getting better in your riding, so there is the chance of outgrowing the potential and abilities of any horse you get right now. If you lease the right horse for you now, you can grow all you can with him/her, then lease a higher trained horse later on when you've surpassed this one's abilities. It ensures that the horse always has a home and that you aren't having to sell a beloved horse. This is especially important for younger riders.

Ask plenty of questions before entering into any contract. You'll want to know who will be financially responsible for what, especially for basic upkeep of the horse (board fees, farrier fees, vet emergencies, etc.). Some owners will require the person leasing their horse to take lessons in a specific discipline, or maybe with a specific trainer. Some owners need to use the horse on specific days, so see if their schedule fits with yours. They'll want to know a lot about you, too. Your schedule, how long you've been riding, your experience level, your discipline and training, etc. Don't jump into anything, make sure the circumstances are right for both parties!

It's difficult to say how much because it really depends on who you're leasing from, whether it's full or part and where the horse is kept.

If it's full lease, then you are required to pay for most if not all aspects of care - so you'll be paying the livery, farrier, vet, feed bills etc.

For part lease, they may divide expenses in half or ask you to pay a portion of the livery, it would depend on how many days you're using the horse.

Some riding schools have horses for full or part lease as well so it's worth asking around even just to get an idea.

I would never loan my horse to anyone. Who knows that they might do if they weren't as experienced enough to not mess up it's training.

Ask all the questions you care to

Once you sign the contract. That's the answer that counts in court.

If it's not written it did not happen

is this YOUR horse or you are wanting to lease on for yourself? I wouldn't do either one.