Good question...I believe the answer is more than would make it worthwhile to own one...unless your Jay Leno or something

Out of curiosity I'm going to see if I can figure out how much they cost now!
Well, it looks like the software/temperature probes/air/fuel gear will cost you about 3500 : http://www.competitiondata.com...e.pdf
It seems that you cannot find out the pricing on a dynojet either unless you call them so now I direct you to:
http://www.dynojet.com/auto.shtml
One other thing to note is the necessity with some models(especially if it's going in your garage) of making a pit for it to sit in, its a rather large assembly with the drums and all. making a concrete pit that large in your garage is not a cheap proposition.I estimate it'd probably cost you 15-20,000 to get totally set up.