Well, unless they are willing to cough up the points to make it a permanent base, then I would say make them pay through the nose for it money wise, and the first time some supervillain attacks their base, it's most likely going to go to the bottom. Now if they were willing to actually pay for this with character points, then I would give it a little more leeway, other than that it just ends up being expensive scenery. And we all know how long scenery lasts in the comics now don't we:D