Walmart offers the Majik Over-the-Door Speed Bag Trainer, model no. 1-1-28424, for $14. (Amazon charges the same with free shipping for Prime members.) Opt for in-store pickup to avoid the $5.97 shipping charge. That's $6 off and the lowest price we've seen. Designed for kids, it features an adjustable height, electronic timer, and stadium sounds.
Note: From Amazon, it usually ships within one to two months.