Like Sheringan, I bought a 512K RAM expansion for my A500 in early '89 and it cost about £90. (There was a RAM chip shortage in the late '80s, I think.)

I got my external floppy at the Commodore show in Hammersmith, November '89 also for about £90. (How can I remember all these useless details of my life and yet forget where I put my car keys five minutes ago!?) (I have actually still got a Commodore mug I bought there, although the red bit of the C= logo has faded to nothing after all these years. My wife thinks I'm very sad for keeping it...)

Anyway, floppies... Verbatim DS/DD jobbies were £1.35 for one, obviously cheaper by the multiple of 10. Or you could get no-name unbranded disks for about 80pence each.