My initial response is "Not gonna happen". I haven't seen anything that size below $1500 though it's been since I made the original post since I looked, so anything's possible.
For anyone who cares, we bought the Microtek below and ended up being really disappointed with it. Slow, mediocre quality, non standard (ie doesn't work with any TWAIN plugin).
Luckily the demand for the large format was so great that we were able to get a second scanner. This time we got the Epson Expression 10000XL and have been wildly pleased with it. It's everything the Microtek is not (quick, high quality and works prefectly with all our software). Highly recommended.