Esportsonline via eBay offers the MacGregor Official Basketball, model no. MCX35WID, for $6.45 with free shipping. That's the lowest total price we could find by $2, excluding the below mentions. This 29-1/2" rubber basketball features a wide-channel design.

A close price: Walmart and Amazon have it for $7, the former via in-store pickup, the latter as an add-on for orders over $25 via Prime.