I’ll upgrade when the price of the next windows product is under $30.
Do you presently have a legit licensed copy of Windows 7 or Windows 8.1? If so, your upgrade to Windows 10 is FREE if you do it within one year of the release of Windows 10 (which is targeted for mid-this-year). Really. FREE.
Other than that I haven't seen precise costs for each edition of Win10, but they should be out fairly soon.
I’m hoping that 10 will be good enough I don’t have to agonize or go to extremes to keep a running version of 7. My 7 machine is a few years old and while I have had excellent luck in having machines last without hard drive crashes, it is always difficult to have to change, especially with so many newer machines changing the BIOS/boot methods that it can be difficult to transfer an older OS to a newer machine.