The actual upgrade requires not much more equipment than a PC and some software (for '94-on Trionic upgrades, anyway). However, it's not just a couple of tweaks but a complete remap of the system and as SteveN says, there is an enormous amount of development behind a well-engineered upgrade. This is especially true of the T7 and T8 systems, which are much more complicated and more difficult to understand and modify successfully.
A good analogy is Microsoft Office (or any other piece of commercial software). It probably costs substantially less than £1 to press a Microsoft Office CD. Why do you pay a lot more? Setting aside any arguments about Microsoft raking in the millions, it must be quite apparent that it costs them a lot more than £1 million to produce 1 million copies of Microsoft Office, given the huge amount of development (and marketing, distribution, etc.) involved.
And on top of all that, they need to make a profit or they might as well not bother.