As above Baiter is about £9 for the day, unless very early in can be a bit of a scrum lauching and recovering, jet ski's seem to get in the way. The slip is long and shallow, so have plenty of help as you will almost certainly need to unhitch the trailer and push in/pull out the trailer (long rope helps, but others will foul it or fall over it).
Cobbs charge about £45, but is a good slip and none of the above applies.
Rockly only really usable about an hour either side of low water, dont know the charge.
Depends how far out "around Poole" extends, another good slip up the river in Christchurch, with only a charge for parking (around £10 for the day). Again be careful at the river entrance at low water.