Amazon offers Prime members the Fisher-Price Kids' Grow With Me Trike, model no. P6831, for $19.80 with free shipping. (Non-members can spend $35 or more to bag free shipping; otherwise, it adds $10.92.) That's the lowest total price we could find by $15, outside of the mention below. It features an adjustable seat and pedals.

A close price: Walmart offers it for $20.09. Choose Site-to-Store pickup to dodge the $4.97 shipping fee.