For a reasonable quality blade, then an average of about ten pence, not including postage.
I say reasonable quality because I like Gillettes, I'd imagine many can average a lot less than this if they like say, Dorco or Lord or something. And conversely I think you average around 20p a blade on feathers, IIRC.
^^ Good point on quantities, I tend to buy quite a bundle at once, at least a hundred anyway, it would seem the price goes up a lot the smaller the quantity.
Is it just me or have prices risen recently? Feathers seem to be averaging 26p or so on eBay, and Gillette 7 o'clock yellow are more like 15p whereas I've always paid around ten.
Even allowing for price-quantity breaks it's still wise to shop around - 5 Wilkinson Sword can cost you anything from 59p to £1.99... for the same 5 blades.
I would say around 30p. Israeli Personnas are £1.89 in ASDA for 10, WS Classic £3.98 for 10 in Boots. Superdrug's blades are up from £1.99 to £2.49 for 10. But factor in the price of sample pack blades and that brings the price down - around £7.50 for 42 blades. Around 18p each.