The average cost of car ownership in the US today is $750/month - including gas, insurance, maintenance, depreciation, etc.
I think that is a reasonable threshold cost for at will utilization of a shared AV for anyone - including a person with disabilities that might require a specialized vehicle or with specific accommodations.
A "regular" shared AV might be used 80% of the time and have minimal downtime between users and minimal dead head miles traveling from one user to the next.
For a specially equipped AV for disabled individuals there are two concerns - the higher initial cost of a vehicle with specialized interfaces and equipment - and the lower expected utilization between riders (fewer trips, further distances between trips). These both will substantially increase costs. In my experience in metro Phx - the difference is about 4-5x.
So if we agree that people will give up their cars if they can get from point to point and spend $750/month or less and utilize a shared AV, and that the cost of a shared AV equipped to safely transport people with disabilities, would be, say, closer to $3000/month. The difference will have to be covered somehow.
There are generally two choices. It could be a tax on all shared AV's - raising the cost to maybe $760/month (this is how mass transit works - everyone pays a little more to cover the higher costs of accessible equipment or paratransit, etc.) or the government just subsidizes it. Of course the government pays for the subsidies via taxes - so in the end - the result is essentially the same.
Overcoming the barrier to higher costs for shared accessible AVs is a critical step to full deployment. It is only "fair" that every pays the same amount despite differences in the costs of providing services - but the logistics and methodologies are still to be determined.