Structure upkeep

[DM B]

Structures weer added as an afterthought, but are very popular and have been much expanded upon (with more structures to come).

As a result I've given upkeep (and to a lesser degree construction cost) an overhaul. Expect to pay somewhat more for larger structures. As a rule of thumb build cost/40 = upkeep (but that can very somewhat depending on what benefits the structure provides).