It varied, if you went to Boots for C64,Zx spectrum, Amstrad CPC 464/6128, top games were £9.99 although Virgin Records sold them at £8.95.
The C64 and Amstrad were usually £1 more than the ZX spectrum.
This is how it was for premium software for 8bit machines, you could also get games on the cheap from Mastertronic, Firebird, Bugbyte, Kixx etc.
These were usually in the £1.99 or £2.99 catergory although some were ok most were crap.
Some premium games came into this catergory when they had been out a few years.
As for the 16bit machines like the Atari ST and Amiga they normally retailed at £19.99 and £24.99 for the premium games although you could get cheaper titles like Barbarian from Psygnosis which retailed at £7. I remember this is as if it were yesterday and i have bag fulls of old games in my attic in there original boxes along with receipts also!
I also have machines that were never released in the EU like the PC Engine, Sharp X68000 and FM Towns Marty in there original boxes and completely unopened.
Let me put it this way, games were more expensive back then than what they are now even with inflation, ive spent thousands upon thousands on games and hardware through the 1980's and 1990s to present day, it was more fun back then also getting stuff on Grey import long before the internet existed for the mainstream public. You really felty like you had something special that no one else had and you had to do you homework to acquire it. a lot of the fun has gone out of it today. I was definitely more hardcore back then than i was now, somedays i would just game all day with no food or water and come away with a headache lol.