Anything from £12 for an RFT one up to £95 for a Telefunken one. A standard Mullard is £40 while a special quality M8136 Mullard is £60. These are all Langrex prices on eBay (they're fixed 'shop' prices, not auctions, so I take it I haven't infringed any rules). It's purely a question of supply and demand.
EDIT I see PM Components didn't sell them all back then
http://pmcomponentsnos.co.uk/epages/...Products/CV491 
.
Price inflation since 1984 is a little over a factor of 3
https://www.bankofengland.co.uk/mone...ion-calculator. so your 55p back then might be something like £1.70 now.
Cheers,
GJ