That's ridiculous!.. IF I was going to upgrade (and I'm almost certainly not) I would sell on ebay or possibly gumtree (not sure if that's an international thing so if not it's like craigslist).Microsoft UK will give you a whopping £100 off if you trade in your SP3 for an SP4!
CeX will give you between £229 - £333 dependant on model.
This. Have a search agent for the past two weeks so I know the range that mine will go for. As soon as I have a tracking # for the SP4 I'll put my 512GB SP3 up, start uninstalling programs, do a disk image and backup data both.EBay. Already have an s3 as a backup and 2 nexus tablets to back that up.