After the cost of purchasing your 3D printer, you'll only have two ongoing costs for terrain printing:
PLA filament can be bought for as little as $20 USD for 1kg, and that is pretty much enough to cover an entire table in terrain. The amount of filament you use for printing models depends on the size of the terrain, but most small buildings can be printed for under 100g of filament.
When printing your models you can set your printer to use less filament by reducing the infill. An infill of 100% would print solid plastic, and be quite expensive filament. We generally use only 10% infill when printing our terrain models, which fills the inside of models with lots of triangle structures, to keep the strength of the structure while still saving filament.
This would print a LOT of terrain!!!
Running your printer requires electricity, but the cost is fairly negligible. A 3D printer with a heated bed would consume about as much power as a desktop computer.