I just reread the original post, and let me add this. I haven't often seen a "premium" for slips over 40 feet. Mostly, it is just paying whatever the rate is, per foot.
Where there sometimes turns out to be a premium is that one often gets charged the larger of the boat length and the slip length. So, the best deal one can get on a slip is when one exactly fits into the slip, or fits into the slip with the allowable overhang. In these cases one pays only for the length of one's boat.
In some sense the worst case is when one is just barely too long to use one size of slip -- and there by needs to pay for a bunch of "unneeded" length for the next biggest slip.
As it happens, I, personally, am okay with "paying extra" though. I feel more exposed to risk when I hang out of a slip, and I appreciate having some extra space in my slip if I want to drop a dingy in for a while, or push it farther from the back of the dock to dive under the swim platform, or whatever. So, although, personally, I'd rather be in "exactly" the right size slip, I don't want to "hang out" and don't mind paying for a little extra space.
Sometimes I've seen situations where the per-foot rate does change with the size of the slip -- longer slips cost more per foot. The reason for this is usually that the beam of the slip is also wider and the distance between the rows of slips used to turn the boat into the slip is also greater (to allow for longer boats to more easily turn). So, the per-foot rate isn't higher for the same space. It is higher because one is getting more beam and more room to turn.
As for where the "cut offs" in terms of the price-per-foot going up -- there isn't a standard. It all depends upon the marina. At my current marina, all slips (except trimaran slips) have the same per-foot rate, making the longer ones, in some ways, a better deal, because they also have more beam.
If I remember correctly, and I may not, at my old marina,. the 36' slips were less expensive per foot than the 55' slips. So, a boat up to 39' (3' could overhang) could get in at a cheaper rate than a 40' boat, both because it wouldn't need to pay for extra feet and because it could get a lower per-foot rate. Of course, it also got less beam, less room to turn, less room for a dingy, and closer neighbors.
Having said that, when they had all of their smaller slips full, but a ton of larger slips open, they often times offered people a deal. They let them have the larger slip at the smaller slip's rate, until a smaller slip opened up (at which time they'd have to change slips or pay more).
The upshot is that, since marinas are configured differently and operate in different markets, they have different rules w.r.t. overhang and different pricing structures. It is really hard to say that "beyond X" the price will definitely go up disproportionately, or "below Y" the price will go down disproportionately. Having said that, as a general rule, less length will be cheaper and a dramatically disproportional beam (which is rare) will be more expensive.
