Good website for general construction/renovation costsDecide whether to do it yourself or hire a contractorNot free, but Fine Homebuilding have great articles that are geared towards construction professionals for this type of thingLike ew2x4 said, make sure the window is an egress, otherwise you can't count that space as a bedroom when you list and try to sell.
Framing isn't hard. Putting up drywall is a 2 person job. Finishing the drywall is an art, and will look like crap if this is the first time you are doing it. Put the drywall up yourself and hire a crew to finish it for you.
Here in Denver, you can take a series of (easy) tests that allow you (as a homeowner) to do your own renovation/construction in your home, without having to hire a licensed contractor (like electrical, plumbing, framing, etc,). You still need to pull permits though. A lot of people do renovations without pulling permits. Its cheaper, less hassle. Some people know what they are doing, some don't. The permits (and inspection process) are there to protect the ones that don't. You can really eff things up if you don't know what you are doing. So read up.
Also, quick story on not pulling permits. A while ago, there was a realtor in a suburb NW of Denver that had completely renovated his home without pulling permits. And then he put it on the market. Someone at the City put 2 and 2 together, went to see the home, noticed that everything was new on an older home, went back to the City offices, couldn't find any permits for the work. So the city made him tear out everything, redo and go through permiting and inspections. Cost him a lot of time and money. So this realtor, really pissed at this point, used his realtoring position to go around and look at a lot of homes on the market on his area. The ones that looked like they were recently renovated, he went to the city, checked on the permits (public record) and reported the ones that had done what he had done. Real dick move.