If you can do a few simple things like accurately measure, use a hobby knife, use a bandsaw, etc. building from plans isn't all that hard. While I've only done it a few times, I've found that some designs are easier than others and some plans are better than others. If you have a few kits under your belt and are able to think your way out of a problem, you should do fine.

As far as cost, I'd say it is at least as expensive as the average kit once you factor in buying the plans, the wood, the hardware, and so on. the cool thing is being able to have something not available as a kit or ARF.

Here are a few places to buy plans:
www.modelairplanenews.com
www.rcmmagazine.com
eBay

I'd suggest something simple like a Stik, Hots or profile plane for your first attempt then move up from there.