I am new to this forum and TR area, so am by no means qualified to offer anything except what I have observed and learned through my own experience.
My wife and I bought a home in Shell Knob a year ago May, but still live in Nebraska. It was important to us to have a slip, for it was a convenient way to get out on the lake, whether in the boat or just sitting on the swim platform enjoying the beauty that surrounds us. In our case, the slip was included as part of the house sale, so making the kind of decision you're looking at was not a factor for us.
I did notice, during the time we were seriously looking at properties, that the houses/cabins that included a slip seemed to sell quicker than those properties without one. That was another factor in our desire to have a slip. Should we decide to sell down the road, it seemed to us that a slip would make the property more appealing to the next buyer.
Having traveled to TR many times over the years, and having seen ads for slip sales on-line and in the local newspapers, I got the impression that, like houses, a slip's value depends on many factors... location, size, condition, age, current marker value, etc. I would think you considered these factors when you bought your house, and would think the same would apply to a slip purchase.
The bottom line... how much is it worth to you? No one wants to over-pay for anything, so you really should check with owners of other slips in that dock or ones near by, in like conditions, to find a fair market value. Local real estate agents can offer some help there as well. We recently re-financed our SK home due to low interest rates. With a full apprasial, they valued our slip at $25k. It's in a 20 year old dock, 10 x 24 in size, and at the time, had an old, worn-out lift. That value came in consistant with what our neighbors told us was the average value for our sub-division. Does that compare with KC area?... don't know.
Good luck. Hope that helps.