I'll just get this out of the way. These are the three questions I get asked when people meet Artoo:
1) What's he made out of? My R2 is made mostly from styrene and resin parts. The body and structure is build from 3 millimeter styrene sheet. I followed plans found on astromech.com. Essentially, It's like building a gigantic model kit where you need to hand-make all the pieces from scratch.
2) How long did that take you? The build took around 9 months. Which I understand is pretty fast for an R2 build. It's a project made a little easier because countless others have done it before. Astromech.com – the site of the R2 Builders Club – contains tons of member build logs and blueprints to build a screen accurate R2. It's also the most open source and helpful online community I have ever encountered. And I've made some great friends with a few local R2 builders that helped along the way.
3) How much did it cost? I honestly don't know for sure. I tried to keep track of costs at the beginning but really lost track, and stopped caring in the middle somewhere. I believe, with all the electronics included, I spent in the $2,000 range to make him. Which again is on the low side for building an R2.
He turned out better than I imagined. Here's some other details: He runs on a 12V battery system. He has full dome electronics and motors in the feet. Learning to program the motor controllers and remote system to get him drivable was not trivial. He has a remote audio system which plays the actual "voice" of R2 from the film. And I've found that he's never really "done". I'm constantly tweaking him.