A finished green, where everything is completely new and original, can run as high as $300. A lesser known sculptor may not charge that much, bringing things down close to $200 or even $150. (this is for a human-sized model)
A model that is a conversion would cost less. A conversion is a previously completed miniature with parts changed, swapped out and often with new parts added on.
And then you have to consider what it costs to have a miniature painted.
I am an experienced mini converter and an amateur sculptor (and a decent painter). As it happens, I was mulling over doing this sort of thing as a sort of "side buisiness". If you want to discuss the scope of what you need, and what it would cost you coming from me, I'd be happy to do so.
I can provide plenty of references and photos.
-Tom
gospog AT gmail DOT com