Prices
Depending on condition, typical second hand prices for a DX7 mk one seem to be as follows:

These prices, accurate as of July 2001, are just for a DX7 mk1, with no additional accessories. DX7S, DX7IID & DX7IIFD prices are around $150-$200 more than a mk1.
Additional, April 2002.
The above prices are probably a little high now, as I have seen DX7's go for around $150 on ebay.
If you are unsure as to which DX7 version to buy, then in terms of features, they are rated in order (lowest first) as follows:
DX7 Original (Brown case, green 'flat-membrane' buttons.)
DX7S Bridges the gap between the original DX7, and the II version.
DX7IID Better sound quality than original, many extra features like layering sounds, bigger LCD.
DX7IIFD Identical to DX7IID, but includes a 3½" floppy disk drive
DX7 Centenial Very rare, silver and gold coloured II version. 76 keys.