It only has value when you can sell it. Make sure it's half and half. There are many rules of thumb to gage what a business is worth. Everyone thinks their method is the correct one. I even offer software that tells you what your business is worth by plugging in some numbers from your annual figures and irs return.
Nothing is final on a sales price until all the negotiations are complete and agreed upon.
You'll have to get nosy and ask for some hard facts and figures from your friend. Feel him out. First, you'll have to have some numbers in front of you, like annual Income and Expenses. It only has value when you can sell it. Make sure it's half and half.