"Amphibious" and "cheap to run and maintain" are essentially mutually exclusive ... but you already knew that.
The Zenair CH701 amphibs are relatively inexpensive; but I don't think that you could run one on a normal C of A.
You might consider an Aeronca Champ, a Citabria, or a Cub Special ... but I think you'll find that the capital cost of amphib floats is pricey. E.g., a new set of Wipline 2100 amphibs (suitable for small airplanes like those I've mentioned) will set you back US$28,500, which doesn't include the cost of shipping or the STC kit.