How/Figuring how much to sell a Land contract (under contract)

[Drake Espenlaub]

Earlier this year I purchased a property from someone who wanted to get rid of it fairly cheap. It is not in the greatest shape, but he got me in touch with someone that had been asking him about renting it. After I purchased it from him I met with this man and his wife, and discussed why they wanted it, what they can afford, and came up with a plan to sell it on contract which would be good for both parties. 

I gave them two payment options and they chose $515 per month for five years or $30,900 whichever comes first, said he would pay extra when he got his tax returns (which I am yet to find out if it is true or not only having it for 7 months). I have had it 7 months and it is good income for me however, I am in a spot where I gave a friend 30% of the deal to purchase it, he has moved to chicago and asked I buy him out, and I have a duplex under remodel I could use funds for. 

Anyone have an idea of how to calculate the sale of the contract? I was thinking 17k to purchase 10k in profit over the next 4.5 years?   If there a more simple way to figure a sale price?