Most of the slips in/around southampton are free, Hythe, Hamble, Warsash,.....
Warsash is the best in term of value, it costs nothing to launch and nothing to park , access all tide barring an hour either side of low tide, still possible with 4x4. Hythe has a concrete slip you pay to park and that's it. Only problem, most of these slip involve a long steaming down to the open sea, unless you only want to fish the solent.