Walmart offers the Summer Infant Finger Toothbrush with Case for $1.77. (Target charges the same with free store pickup.) Opt for in-store pickup to dodge the $5.99 shipping charge. That's the lowest total price we could find by $3. It features a dual-sided brush for cleaning or gum massages.
Update: The Walmart offer is out of stock; however Target still offers it for $1.77 with store pickup.