My reason for moving was I don't want to pay the ludicrous amount that Plusnet are charging for the renewal. It looks like they've effectively held my domain to ransom. They'll probably make the changes now I've paid.
The £100 does not sound entirely unreasonable (or more specifically unusual) if the domain name has expired - generally speaking an expired domain name is more costly to renew.
It also does depend on the domain name type (no idea what it is from your posts) and the length of renewal.
Of course, the expired nature aside you might find someone else is offering better prices normally etc of course - unfortunately as per my other post, leaving it until a couple of weeks before expiry is not advised (and some providers and domain types do not permit transfers in the last month for this reason) - it's best to buy a one year renewal (most people offer this) and then do the transfer - or of course with hindsight, do the transfer a lot earlier!