At our local no frills Marina that is on a feeder creek on Lake Erie a slip for a 17 footer would be around $1000.00 CDN . No power, no clubhouse but it has a Jonny on the Spot. Oh a nice restaurant and bait shop. If I am not using it within a week I have to go and check the auto bilge is working. That can be a pain too and it's only 10 miles from our lake cottage. I haven't found any small solar panels that will recharge a dead battery. If you want power and all the socializing the closest Marina would cost double that or more. For a small boat like that I would pay for launch fees and Marina storage. That's a small boat to launch, the more you practice the luckier you will get. A pal has a 24 foot cuddy he trailers. With the 3 of us all communicating and doing our jobs we can get her off or on the trailer in 5 minutes with the power winch. Now having said that it is nice just jumping in and off we go. A auto bilge is a must if you keep a boat in the water. I know a few owners that pay one of the kids working at the Marina to check the bilge regularly.
Marinas are like Golf courses. Memberships vary based on amenities. Some Marinas are a huge social club with big arse boats parked in slips. Some have areas in front of the slip where you can put up a gazebo type shelter, table BBQ etc. Basketball courts, tennis, indoor showers, pools, hot tubs, fine dining, big club houses for social events, indoor and outdoor bars. You get what you pay for. There would be no reason you would have your 17 footer moored offshore thus requiring a dingy. Boats that have a big draught and can't get into shallow slips require a dingy, not for a 17 footer. Sorry I misread your question. I think it would be as big a pain to moor offshore as it would be to trailer, actually much more work for many reasons like loading the dingy then unloading it, what do you do with the dingy when back on shore? Trailer it home? No way mooring is an advantage in any way.
I call our place Chernobyl Marina because US Steel is across the road and the coal fired Hydro plant to the east, not pretty views. Thus the cheap rate, I need a slip not a private social club. As far as the best time to shop I don't think it matters much unless there are many open spots. Then a deposit in the winter looks nice to a seasonal business and you might work a deal. Not likely here though as there is actually a waiting list believe it or not.
I hope this helps Chris.