Well, like you say "every time I would build my own that wasn't a factor because XP was around for such a long time". As long as you stick with the OS that you have, you only pay for it once. With of-the-shelf systems, you pay for the OS with every buy.
I guess after 2 or 3 home-builds using your non-OEM copy you've recovered the additional cost (if any), depending on what edition you have.
I must admit I'm not looking forward to buying Win7 though; that RC is gonna expire 1st of March.