How much should a couple put away for their autistic child as an adult?

Depends. In 2010 data, the average cost for a middle class family to raise a child from birth to 18 years was $222, 360 (22 % higher than 1960's). This does not include college costs. Depending on the level of functioning, independence-adjustments need to be factored in. Where you live; are you a single parent; what's your income; public vs private school...All will have to be considered.
Options. Average cost for a person with autism is ~ $800, 000 over their lifetime. When your child is 14, the school district will start the process of transition to adulthood, with work programs, alternate diplomas, etc., if needed. I suggest at age 16 you consult an attorney about legal guardianship, financial requirements for eligibility for ssi & medicaid, as (s)he is considered an adult at 18 !