If we're talking game cash price for a non-current system then £15 is a bitter pill and £20 is the absolute limit, but normally I trade for stuff, probably 98% of the time I've either found it for a quid or two in a charity shop or it's traded towards.
There are lots of opportunities to buy something cheap then trade it towards something else, either directly trading for games or trading it for something else which can then be traded for games or swag. The non-direct route pays off.
Three instances: 1) The trade-in shop CEX takes media and electronics, toys-to-life figures etc. and vends games from the latest releases back to NES/MS. 2) a large mail-order specialist I use sells anything from games and systems, magazines, swag, strategy guides, old programming books for home computers, calculators, electronic games etc. etc. I can pick up an old calculator and pretty much trade it for a game. 3) A local game shop changed hands a year or two ago and now specializes in retro games and anything else 'interesting' (Lego, Games Workshop, anything) and at the same time imports Japanese games. I've got about 7 import region-free Xbox 360 shmups from them through trade, on top of other items. These are not things that are lying around in the wild.
So yeah, trade towards, that's how I roll.