Based on my experience traveling solo or in small groups, I think it’s definitely more expensive due to many different aspects. As you’ve mentioned, the main factor is transportation. Due to accessibility issues, available means of transport tend to be fewer and you usually have to rely on more expensive options. For instance, most inter-city buses are out of the question for many people with disabilities. It also depends on the country, but in most places I’ve visited, long distance buses is the least accessible transportation (and usually it’s the cheapest), thus you have to rely on trains (if both the train and the stations are accessible), plane or taxi (which in many places are not accessible or they sometimes charge extra).
Then there’s the accommodation. If you are traveling on a low budget and you plan to stay at a backpackers hotel or at a youth hostel, at least in Europe, these options don’t tend to be accessible. If you’re looking at apartments through AirBnB or similar, the accessible options tend to be more limited. Usually accessible apartments are in newer buildings, which are also more expensive. Also, as you’ve said, in some countries accessible rooms are high tier rooms and are a bit more expensive.
Then there’s the planning time. It’s not direct cost, but planning time is way higher, as you have less room for improvisation and as you need to make sure that everything is accessible or, otherwise, as you have to find an accessible way or option. This, as you know, is time consuming and, in some places, nearly impossible.
Another factor to take into account related to time, is traveling time. Usually, due to the aforementioned accessibility problems with transportation, traveling time is also higher (less options, detours, slower transfers, etc.). As traveling time is higher, either you can sightsee less or you have to stay more days to do the same.
Finally, if you’re traveling with an assistant or someone that helps you, then you have to nearly double the amount.
It’s true that sometimes there are discounts when visiting museums or other sightseeing attractions for people with disabilities and their assistants, but sometimes it’s the opposite, you have to get specific accessible seating that usually it’s not in the lower tier of seating prices either.
All in all, the discounts do not compensate at all the extra cost of traveling. Nevertheless, as also said, extra cost also changes a lot depending on the country, but I’d say that one way or the other, unfortunately, there are still extra costs when it comes to accessible travel.