In general, west of FL should be less expensive. And probably east of TX likewise. There are dozens of places in AL, MS and LA.
Is this for storage
only, or do you want to do work or have work done?
Two places in Mobile, AL are Turner Marine
and Dog River Marina, both on the Dog River (next door) just south of Mobile. Turner is probably slightly cheaper. It takes a day to get up the bay, but they are both much cheaper than places on/closer to the Gulf (such as Gulf Shores, AL or Pensacola
, FL and between).