Sounds like a good area and good starter park providing demand checks out and you have the ability to fill it up.
10 x 250 x 12 x .50 expense / 12 cap is 125k or 150k at a 10 cap plus whatever you value the homes at. If they don't need any work they are gravy money. Realistically you will only get it for under 150 if the area is bad, or any number of other circumstances are in your favor.
If I thought the area was strong, park rents could go up, and I had a plan to bring in homes I would consider going higher, but carefully.
If he has an asking price of 300k already then it's too high as a park.
That's a start anyways.
Need more info really