Well,
If you were model specific, then its reasonably cost effective but if you want to be able to map pretty much anything, it gets expensive, especially when dealing with modern checksum algorythms as you often need 3rd party software to deal with it.

Also depends what quality of hardware you require. Example: That AFR Kit you saw on the other post today would no doubt serve you quite well for a while at £400 ish.. where as my industry standard unit is £6000+vat.

As with many things, the final answer is "It depends" Ive spent over £30K, but i can think of another £30K worth of usefull kit i still aspire to purchase... where do you draw the line? LOL