The construction costs of a new home will vary greatly, depending on the amenities you decide to put in the home (for example, Hardwood Flooring vs. Laminate or Engineered Hardwood, Granite vs. Corian vs. Formica Countertops). For a rough calculation, you can use $150-200 per square foot, depending on your taste. Since things are tight, especially in the contractor world, you might be able to get it for less (I've even heard of some contractors going as low as $65/sq. ft), but it also raises questions of quality and whether they'll be around in six months.
As for approved vs. unapproved lots, that is really a zoning question. Each lot is a town is classified in a certain way, which limits the type and size of structure you can put on it (or if you can even build at all). For example, you can't put a 3500 sq. ft. home on a lot that's 50x100. If you have a particular lot in mind, the best thing to do is for you (or the agent you're working with) to contact the Zoning Officer for the town, who should be able to answer your questions.
If you've any other questions, please feel free to contact me directly. I have a lot of experience working with Home Rehabbers and Construction and should be able to help you out.
Talk to you soon,