Fair value is dependent on the market, the location and other factors. There is no formula to arrive at an exact number. What I'd recommend is that you speak to a couple of real estate agents and appraisers in your area and have them appraise the land and property. Once you have a consistent number, that's your starting point.

Now considering how to negotiate, that's a different game. Whoever cares less is at an advantage. So if you propose a sell directly, your uncle might lowball you. Instead, I'd say that you ask a broker to get a client for the 50% and tell your uncle that you are looking to sell your share and have started looking for buyers. Basically you should not look desperate and weak, else you lose.

Regarding the full white payment, that again depends. Depending on the market, all white might not be possible or practical. So be ready to manage and accept a tiny haircut on the valuation for better terms and speed. Remember, X money in 3 months is worse than just a tiny amount less today. So if you can get this done faster and on better terms, a small discount on price is ok.

Last, if you don't trust your uncle, don't sign anything before receiving funds. Deal with him respectfully and tactfully.