
No-one will be able to give you an idea of the figure until you know more about the plot of land etc. You might find it difficult to even find a plot of land suitable for development that doesn't have outline planning permission already on for exactly the reason that a much higher price can be asked for it if planning permission is in place. A site sold without planning permission may never get planning approval for a number of reasons (and there are many) so it is cucial to know what the likelihood of planning approval will be if you do find such a site before you aprt with any cash.. A telephone call to the local Planning Department might be enough to determine any major obstacles but will not reveal the finer legalities.
Also, with regards to costs, it's almost impossible to guage without knowing more about the likely location of the plot etc.You have 20k to play with, 12.5k of which will be spent on the cabin? Does this include costs to erect the cabin, if not that will be extra. Then add the cost of the plot of land. And if the location is very rural, you may have to pay more to bring services like telephone, electricity, sewage disposal, gas etc. to the building. I don't wish to dishearten you at all but I reckon you could spend 20k+ before you even get anywhere near the stage of installing plumbing, electricity, heating etc
I would advise getting as much advice as possible before embarking on such a project. Building on a previously undeveloped site can be fraught with difficulties not to mention hugely expensive. And do not take estimated costs as gospel...our recent renovation (almost complete rebuliding of an old house) came in almost 3 times more than the original estimate!! Ouch!! (Our bank manager is not a happy bunny :) )
Having said all this, we intend to do something similar in a few years time (but won't have to buy the site as it's being given to us) so, if you do go ahead, I wish you all the luck in the world. Let us know how you get on.