Cot per slip is usually priced by the ft. However, there are many factors. I have a 28 ft. boat and have seen prices for the season range between 1,200 - 5,000 USD. IT all depends on the marina, it's location, it's ammenities, type and size of docks, etc. Floating docks in a well protected area with full facilities, club house, with high end bathrooms, showers, etc. is going to cost more than fixed docks prone to tides, surge, wake traffic, weather, etc with little or no amenities is going to be much less.
Proximity to good boating is going to play a factor. If you're close to open water and still protected in the marina it will be a premium over a marina that takes you 30-40 minutes of running downriver before reaching open water. Restrictions by depth such as a shallow marina or one which can only afford reasonable access at higher tides and restricted access at low tides will be much less. This applies to depth as well as low bridges restricting passage at high tides. Draw bridges and locks are not appealling and if you have to pass locks and drawbridges, it will most likely decrease the slip fees.
The rule of thumb is, you get what you pay for. Decide what it is you need in marina as compared to what you want in a marina.
(BTW-These are generalizations. I understand you may be boating on freshwater and tides, surge, etc. may not be a factor for you. However they do factor into the prices when they apply).