Quite often though the process is reward in itself. You're right you could put a value on the time spent building something from scratch but when you commit to doing something like this I'm not sure any scratchbuilder is actually seriously considering the $ per hour value. Whatever you're doing it's most likely simpler and more cost effective in the long run to buy it, what price do you put on each hobby hour?
I think if you're going down this route so long as you're having fun doing it you should actually be reducing the cost of the model because you're entertaining yourself much as you would going to the cinema or watching a Blu-Ray. If it's not fun then you should definitely be buying it and feeling comfortable about the up front costs.