I agree that it would be awesome to sponsor a fighter simply because having anything to do with the UFC is awesome lol and the demographic of the UFC is one that spends a ton of money.
When it comes to how much they charge their sponsors, that depends on A LOT of factors. My understanding is that to sponsor a UFC fighter, you first must pay the UFC $100,000. Now, you would want to confirm that because that number may have changed or disappeared but I did read that once before. The factors after that include; fighter popularity (Josh Koscheck is likely more expensive than TJ Grant but cheaper than Chuck Liddell, etc), where their fight is on the card with unseen on tv undercard guys probably making the least, and where you want your logo. If you want your logo on the banner and on the butt of the shorts, you're likley gonna pay more than if it was just on the back of his shorts leg. There is no generic formula that Im aware of, but these are the kind of things you will want to think about.
My suggestion to get an answer to your question would be to pick a fighter (or multiple) and pick where you want your logo and find their website where they will likely have contact info for a manager or coach or someone along those lines that handle this type of thing and shoot em an email.
If you do find out the prices for a particular fighter, please post it. Id really like to know.