hi mark
on the whole, boards tend to fall into 3 categories....
1.plasterboard and water resistant variants...
2.backerboards/nmp etc....harder wareing, less impervious to water penetration, dont break down as easy as plasterboard
3. then the next level
..wedi, marmox, dukker, shluter etc. these are the most expensive due to their thermal values, weight, and ultra water resistance
so when you are pricing, you will pick a board from the above category which is satisfactory for the cost and the customers requirements and your availability!!
for ex. you may reboard a bathroom in plasterboard but just tank the shower and bath area..this will be suitable and less expensive as doing it in backerboard and tanking.
then on the other hand, a customer who wants good thermal values and full water protection will insist on wedi etc....its all down to cost. you will get to know a product an know its cost and its you that will decide..
nmp and backerboards are similar but a little different regarding their fitting requirements..if nmp is cheaper and readily available then you might find thats the best option but if mike tt has access to backer board, he'll go for that...
what i will say is, thermal issues are becoming very important and people with money who are into preserving the enviroment will want to know about thermal boards as gooner is stating....then again if theres a family on a low income and it the kids trainers or wedi board, in all fairness they are going to want to only pay what is important to their budget and current prorities....
hope this helps a bit mate
ed