Another angle is to look at it as a straightforward investment, the same as any other.
If it pays an income of £2,000 p.a., and incurs upkeep (weed cleaning, mowing) costs of £500 p.a. say, that's a net income of £1,500 p.a. Then insert the 'yield' value of your choice; let's say 5% for now. That indicates a value of 1500 / 0.05, which equals £30,000.
Then adjust upwards or downwards depending on whether you think the income will go up or down in time, whether a change of use to a caravan park could get you more profit, etc etc.
Does that help?