The only cameras where you can change sensor are medium and large format. The cheapest alternative so far is the Mamiya 645 AFD II with the Mamiya ZD digital back (22MP). The camera body costs $3000 and the ZD back $7000. Lenses are extra
Not exactly cheap, but around half the price of a Hasselblad or Rollei/Sinar/Leaf combination.
Many photographers are dreaming about upgradeable digital SLR's, or even better: digital backs for classic cameras like the Nikon FM3A. Unfortunately, it's very unlikely that something like that will appear. It's simply cheaper to mass-produce complete, new cameras. The cheapest ones, Nikon D40 and Pentax K100D are now below $400.
There was actually one DSLR with exchangeable back: The Leica R8 and R9 (same camera, but different electronics) could alternate between film and the DMR, a rather big as well as expensive digital back, and the DMR is now discontinued. You can still buy the R9 tough, but that's film only these days. Leica is expected to launch a new, all digital DSLR in the not too distant future.
