How long is a piece of string? EDIT: Obviously great minds think alike hey herrods, I was writing this as you posted.
You can spend anywhere from a few hundred dollars to build your own computer, all the way up to a good 3-4k for a fully sequential completely optioned motec.
Good thing with buying an aftermarket computer is that the computer stays with you even when you change platforms. I started with an L28et, now I am moving to an RB and my computer is the same computer, just a different loom and tune.
I had a shagged out engine and was putting a lot less timing into my L28 then I thought and it was still a hell of a lot of fun. One day I think I will go back to an all out L28et, but for now I am trying something else.
I also have some L28et parts for sale in the "Parts for sale forum" if your interested, although postage to melbourne would be a killer since they are the heavy items like manifold, turbo, intercooler, gearbox, etc, etc etc.