Unschooling is priceless. It cannot be bought. And "cost" is a difficult concept, so if you have an easy answer floating to mind, try to scatter it and look from many different angles.
If a child is in a private school, unschooling won't "cost that much," meaning no one will send you a tuition bill and a steady stream of fundraising requests and tell you what clothes and shoes you have to buy.
If both parents are working and decide one should quit work and stay at home with the children, will it "cost" a full-time income? In one way of looking at it, perhaps. But counting potential is a trap.
If a family values love and relationships, unschooling can pay off in a jackpot of closeness and joy that could hardly be possible with school in the equation, and could never be bought back with a thousand hours of expensive therapy down the road. (Maybe factor in the time savings of not spending a thousand hours sitting and talking about what you could've done differently, in addition to the cost of it.)
There's more: SandraDodd.com/unschoolingcost
photo by Sandra Dodd