When the British say that something cost "5 bob" how much is that?

Bob is slang for shilling.


http://www.24carat.co.uk/commoncoinnamesframe.html

a bob was a shilling, which is now 5p which is 1/20 of a uk pound, so it depends on the exchange rate

'Five bob' is still used to indicate one quarter of a pound, in the same way as 'bob' is still used to indicate 5 pence, especially the 5 pence coin. (The 5 pence coin is the same size and weight, and has the same value, as the old shilling coin which it replaced.) The most common expression where bob is used is "... I'd bet a bob that ...", indicating the speaker's near certainty that something is true.

Lammas - I think you're a bit out of date there. The original 5p coin was the same size as a shilling, but the current one is more like the old silver 3d coin. No-one that I've ever heard (since the actual shilling of 12d went out) refers to 5p as a 'bob' or a quarter of a pound as 5 bob. And I've never heard anyone betting a bob either. Quid is still in informal use for pound, p for penny or pence, and guineas only in horse racing. Bobs, tanners and so on are long dead. Crown is used for a non-circulating 25p coin (legal tender but no-one uses them), but only collectors who thought they'd increase in value have them. (As with most things produced for collectors, they didn't...)

Basically, 5 bob WAS 60 old pence, a quarter of a pound. Not nowadays, it ain't. Many people who have grown up since decimalisation don't even know what a bob was. One of the few places where you will hear the term in use is Blists Hill in Ironbridge. There, only pre-decimalisation coinage is valid in the shop. You have to change modern money - but at an exchange rate reflecting the current value of things. You won't get 60 old pennies for 25 new ones - because the prices are the old ones from Victorian times. Inflation has to be taken into account.

Reference for Blists Hill -
http://www.ironbridge.org.uk/our_attractions/blists_hill_victorian_town/

5 bob = 25p

2 1/2p = tanner
5p = "bob".
2 bob = florin
half a crown = 2 bob and a tanner
10 bob = 50p
quid = £1
Guinea = £1 and a bob.