Libre / OpenOffice etc – okay if you want “okay” compatibility, and don’t have to deal with anyone using modern versions of Office, especially Excel, and aren’t bothered with any formatting looking different in real Office or broken the other way, and if you don’t mind it taking an age to start. Basic stuff it’s fairly compatible. Complex spreadsheets from my accountants however do struggle and are messed up royally saving back to an Excel compatible format. Presentations you prepare and then load in PowerPoint on site can be very embarrassing! (I talk from experience). Oh, and the UI is still based on Windows 95 😉
Other than that, as suggested above. Also note an Office 365 Home subscription includes 1TB of OneDrive storage, although business versions (and maybe academic?) don’t give you that as personal storage. You do get business storage though but that goes to the business. Just something to consider if you get Office via a company you work for that isn’t your company.
If you’re self employed with a one-man (or very few) business, also look into Microsoft Action Pack. Now this isn’t under the “cheap” bracket for Office itself at some £300 a year, but it includes everything you need to run the IT of the business. Multiple licences for operating systems, databases, Office, servers, Azure, and if you’re a softy into development you get development software also. If you use it all it’s well worth the money. If you just want Office, then not.