The cheapest way to get a float plane is to put a set of floats on a plane you already have. If you are looking for a low cost seaplane, the Neptune both 49 inch and 60 inch are available now for 69.00 and 79.00 plus shipping from Value Hobby.

http://www.valuehobby.com/airplanes/49in-neptune.html
http://www.valuehobby.com/airplanes/...dy-to-fly.html

You can get a 46 to 52 Asp engine from Hobbypartz.com for aroun 60 - 62 dollars.

That combination will probably be the cheapest fuel set up you will find new.