Doug has the money to buy the group home, sure, but only in the service of making the problem go away (put the autistic kids in a different neighbourhood, so they don't end up leaving the house in Ward 2). The cost of providing care to autistic kids is evidently 'gravy' to the Fords and I wouldn't be surprised that, not having to deal with it themselves, Rob and Doug don't see why money should be spent on it. Randy presumably doesn't want to either.
On top of that, there's the possibility that, in line with a certain segment of the tinfoil-hatted talk radio demographic, the Fords might believe that autism doesn't actually exist.
