For several years know I have felt that Disney has raised prices at the restaurants and to the dining plan to give more "value" to the Free Dining incentives. It also discourages non-dining plan people from using table service restaurants so there is more availability to dining plan customers.
They would never want to discourage anybody from using table service restaurants, not as long as there are empty tables.
When pricing their menus, they take into account several different types of customers:
1-- Those who stay on resort, on the dining plan -- Expensive prices make them feel like they are getting more value. So as per your theory, this encourages them to raise prices.
But it doesn't end there.
2-- Customers staying on resort, who are NOT on the dining plan. These guests are somewhat trapped. They generally are relying on Disney transportation, and therefore have to eat all their meals on property. Just like getting food at a ball game, since they have you trapped, they can increase their prices for these guests. But, only so far. You want guests to enjoy staying on property, and returning for the experience. So making them feel like they were miserably ripped off with the restaurant pricing, isn't going to encourage them to return. So while for these guests, you can somewhat justify higher prices, but only to a degree.
3-- Off-site guests, including local day guests. As many as 2/3rds of visitors are coming from off-site. So you really can't ignore these people. For these people, there is pressure to drive prices DOWN. It is convenient to have your meal at the park, without leaving the property, and that can justify slightly higher prices. But by only so much. Prices need to be low enough, to discourage locals from just packing their lunches at home, or discourage people with cars from taking a lunch break off property. Somewhat reasonable prices encourage these guests to stick around for dinner... or buy some extra snacks.
A financial expert could tell you the overall effect. I'll just give you my own conjecture --
Combine all these factors --- Conclusions:
1. Character meals. Disney can charge almost anything they want for character meals -- Due to their limited availability, and the fact that you can't really get them off property. It's more of a charge for an extra attraction, than for food. I'm sure there are people who wouldn't hesitate to pay $100 per person to eat with Princesses at the Castle, even if they served nothing but chicken nuggets. As a result, we constantly see character meal prices escalating significantly in price.
2. QS meals and snacks -- Can only sustain prices slightly above off-property comparison. If a QS hamburger is $20 -- Your casual day guests are going to completely balk. If an ice cream cone is $9 -- You're not going to sell any ice cream cones. Thus, I think it will continue to be rare to see QS entrees that go above $10. You also need to keep an affordable meal option, for those who feel out-priced by TS dining.
3. In-park regular TS dining -- As long as they fill up, they can get away with pretty inflated pricing. But still want to keep the pricing reasonable enough that your day guest decides to stay the extra couple hours for dinner, instead of packing up at 7 pm because onsite dinners are too expensive.
4. Resort located dining, especially signature dining -- This is an entirely different class. During slower times of years especially, these restaurants don't just get park guests. Local diners will use these restaurants, even without stepping foot in a park. They can only elevate their price so much, for "Disney magic." While they have some customers who are locked into Disney property, they aren't locked inside a park. Pretty easy to get a meal delivered to the hotel, or take a cab off property from the hotel. can even go to "off site" restaurants with Disney transportation --- The Swan/Dolphin restaurants. So these signature restaurants really have to be somewhat similarly priced to signature restaurants off property. Sure, there is some inflation (especially the $70 lobster at Narcoose's). So a $30 off-site entree may become a $40 signature entree, but you aren't going to see ridiculous levels of inflation.