RE: cheapest bi plane
The only one that I can think of that fits your needs in a 40 size is the Great Planes Ultimate. It runs $109.99 at Tower Hobbies. You can find the plane here [link]http://www2.towerhobbies.com/cgi-bin/wti0001p?&I=LXJ581&P=0[/link]
Hope this helps
Ken