Right now you can find the CST Berger 18-90N Computing Function Digital Roller Planimeter on sale for $393.39 at Amazon.
Ah, the Planimeter. I first found out about the Planimeter when I picked up an old-fashioned analog one at a garage sale. I guess the easiest way to describe it is that it’s an instrument that will determine the area of an irregular space through mechanical integration. There’s some good stuff up on Wikipedia as well as these two pages (#1, #2). You can still buy analog Planimeters — we found one new for $174.
The digital Planimeter certainly has some advantages; it will measure up to 10 square meters at 1:1 scale (often times planimeters are used in measuring off of maps, such as lake area) and claims an accuracy of +/- .2%. It also converts between units, which should eliminate a lot of calculation errors.