As title, I've never used windows on a mac before, and I won't use it much, so don't want to spend much.
Besides the purchase of a Windows 7 key, you have to buy software to allow virtual machines on your mac, like Parallels or VMWare Fussion.
That's true, I completely forgot about it, maybe because it's not possible to use OSX and Windows side by side, but given the OP wanted the "cheapest way", it certainly applies.OP wouldn't need any additional VM software if they just use Apple's Boot Camp (as long as they don't mind rebooting into Windows when they need it).
Purchase a Windows 7 key.
Download windows 10 and use with Windows 7 key to activate it.
Where would you recommend getting a scam-free, supported copy of Windows 7 Pro? From my poking around, the path to obtaining one seems to be pretty sketchy (unless you buy retail, which comes fairly close to the price of Windows 10 itself, eh?).
Try this site for key http://www.vip4sale.com/ they appear legit and offer prompt serviceAs title, I've never used windows on a mac before, and I won't use it much, so don't want to spend much.