Chili - exactly. It's first and foremost a business transaction. First you need to value what the share is worth. For that you need to know the value of the aircraft and any other assets and liabilities (mortgage anyone?) that are part of the share you intend to buy. Then you decide on an offer price.
Best advise here is if you don't know how to read a balance sheet, value assets and liabilities, make sure to get some advice from someone who does! That would mean a licensed mechanic to determine the condition of the aircraft, and an accountant of some sort to take a look at the books.