We used to live at the Harborage Marina in St Pete before we left to go cruising for 2 years. As of last May, the rate was $1000/month plus electric for our size boat, a 50 footer and they had slips available. It's much closer to SpringHill then Sarasota.
PS: We checked out Regatta Point and Twin Dolphin mentioned earlier. They were both around $1500/month.