Depends on how old the plate is, or whether it has some other set-up like a name (without 'doctoring' the digits).
I was offered £5K for mine by a dealer (that's what they said I could get - but they may have sold it for more).
Have a look at this website for more of an idea:
Regtransfers.co.uk ::: Your Search Results
PS: Sorry, that link only takes you to their general page. I did enter 1 ALT as a search and it came up with £30K.
PPS: Remember; if you do not space the letters and numbers correctly then you could face being pulled off the road as your vehicle is not 'road legal' and in the event of an accident you may be held responsible, even if it wasn't your fault, and you could find your insurance is void.