My Grandson has been in a wheelchair or a special stroller since he was 3 months old. This is the company that my daughter got her vans from. They list some prices on used and that may give you a ballpark idea. She also got financial assistance but I am not sure who she got it from. I will ask her later today when she gets off work and edit.
clickyhttp://www.advancedmobility.net/express2.0/results.php?location=austin
My daughter said the dealer helped her locate a grant plus Toyota has a small rebate for the conversion.