Sorry Andrew but got to point out that Jo's figures are correct.

One Off Joining Fee to become a member = £400
Annual Subscription = £100
Becoming a Tag Holder = Free (but you need to get a credit account with Nominet so they will need to get some trade references from you).

Basically you CAN get a Nominet TAG without becoming a member but domains then cost the full £80.... if you are transferring domains in for example.... you wouldn't have to pay for them until renewal was due, but then they would charge the full £80, at which point it makes more sense to become a member and get them for £2.50 / year.

I think you may have confused "min £500 credit". That means, Nominet give YOU a £500 credit facility.... so you can register up to £500 worth of domains within one billing period. Not the other way round.

Cheers