If we were to incorporate a charity or nonprofit of some sort in order to fund a small holiday resort or intentional community for autistics, how much would you pledge on a monthly basis?

Myself and Amy have been looking into precisely this. In order to know how realistic it is we need to know how much people would be willing to pledge.

We are also investigating things such as housing benefit for permanent residents, though that would only be possible after sufficient funds are raised to purchase a property.

Essentially, it would work like this:

Fundraising period during which people pledge X amount of money per month and this is placed into a suitable high-growth investment fund of some sort or a generic savings account.

Once sufficient funds have been raised, purchase of the property and any work needed to get up to decent standards.

Finally, ongoing maintenance expenses paid through voluntary pledges and/or rent payments from residents.

Note that this is more likely to be a nonprofit or social enterprise than a registered charity due to regulatory burdens upon a charity, but it will definitely have strict bylaws defining how money raised may be used.

An example:
1 person pledges £200/month = £2400/year
5 people pledge  £100/month = £6000/year
10 people pledge £50/month = £6000/year
50 people pledge £5/month   = £3000/year

That's £17400/year if we achieve the above goals if we ignore interest.
After 3 years, that's £52200. This is more than enough to use as downpayment for a mortage.
At this point, if people maintain their pledges there will be £1450/month available. But assuming a drop in pledges, let's take that down to just £1000/month.

So, we have £52200 in the bank and £1000/month to spend from pledges.
Let's round down how much is in the bank to £50000.

A deposit on a mortage for a £150k property of £50000 gets a mortage from HSBC that costs roughly £998/month. (this is according to their site, with an APR of 4%, rates may be different for business but it gives a rough guide).

There are properties in this price range in wales that can house about 12 people (and do so nicely too).
So, if we maintain those £1000/month pledges but charge each of the 12 people £50/month rent on top that would leave £602/month for general maintenance costs of the property. And of course if rent goes up to £150/month (that's £37.50/week - cheaper than a lot of places), then £1800/month is available for general maintenance costs or for mortage payments if the voluntary pledges cease.

1 person pledges £200/month for 3 years
5 people pledge £100/month for 3 years
10 people pledge £50/month for 3 years
50 people pledge £5/month for 3 years

Then 12 people pay £37.50/week (£150/month) for however long they want to live there.
Once those 12 are in place, another fundraising cycle can begin to setup another property.