Interesting point, years I ago I did a thread using an inflation calculator to work out the relative price of singles, it was quite a surprise.
Here we go: http://www.thisismoney.co.uk/money/bills/article-1633409/Historic-inflation-calculator-value-money-changed-1900.html
So in 1976 a new single from Dub Vendor's stall would have cost (I believe) £1. That is now £6.10.
For many years the price was fixed at £3, but lately its shot up to between £5 and £9 for a new release. We're still paying roughly the same as we were then, relatively, and for years we were paying much less. Any way you look at it £18 is a lot of money to be asking for a 7" though, high production/import costs or not.
12" discomixes were £3 in the 70's, and £5 seems right from memory for 1991. In relative terms today that £3 would be a wopping £18.31, so we're getting a bargain at £9 a shot
How many of those could you have afforded at £18 per week? Not many