Yes, it is actually. Using forks you can have it in your script to require certain materials & money in order to upgrade a certain weapon. It isn't that tough; although, there are more complex ways of doing this...the general idea behind is something like this:
IF PARTY POSSESESS "Copper Ore"
> Wait 0.0
IF PARTY POSSESSES "$300"
> Message: "I can strength your weapon now, ya losers."
> Message: "You do not have enough materials! Get out of my store!!!"
A bit lazy on my part, but something like this should be okay... ^^