It is pretty cheap to build, the only part which costs anything at all is the connector. This is a 22 way 0.156" Single sided connector. I used a Farnell part which is a 22 way double sided connector with solder tags, which worked fine, and built the circuit on a small piece of stripboard. The wiring is messy, but the circuit is actually simple enough.
When dumping cartridges, you need to disconnect the internal anti-static shield which is in the cartridge. The best way to do this is to shove two dowelling rods of the requisite width up into the holes in the cartridge. This is what the bars sticking out of the console are really for.
Then plug the cartridge in, and run the dumping program. This expects the dumper to be on LPT1, and has a timing delay which works okay on my 700Mhz Athlon. It saves the file as "ROM.BIN", a 1024 byte ROM Image and also compares it against the Studio II "Tennis" ROM, which you can use as a testing item. I chose this because it is the only common ROM image (I think).
If you go back to the index and look in the utilities there is a program to build .ST2 files from a binary dump and a ROM image. Examples of these can be found in the game downloads.