12 lots x $240 rent x 12 months x .5 (net of expenses) x 10 = $172,800. You then add in the value of 8 mobile homes (let's assume $5,000 each but it's totally based on the age, size and repair of each unit) so it's maybe worth around $212,000.
The only way you can hit the seller's price is if 1) the homes are worth more than $5,000 each (but I doubt that based on the age) 2) the lot rent is significantly below market and has a huge amount of room for increases or 3) the seller will carry the financing with a tiny amount down, non-recourse for a 10+ year term.
I think I would offer this seller $200,000 and see what happens. If you lose it, I don't think it's the end of the world, as it's not the most compelling deal I've heard today.