This Talos software and sensor box were discussed over at homebuiltairplanes.com and look like a very attractive option.
If you don't need the gyro functions, or can accept the lower performance of an Android tablet or Apple iPad's built-in sensors, an even cheaper option is the Blue Fly Vario.
http://www.blueflyvario.com/Originally just a Bluetooth variometer for gliders, there is now an airspeed option as well plus GPS if your tablet doesn't have a proper one.
Cost is less than $200 U.S. dollars (prices on the site are in Australian dollars) so with a nice Android tablet, some software and a few bits and bobs you'd still be at well under $500 for altimeter, VSI, ASI, GPS w/moving map, slip/skid and compass functions.
