Cheapest Airplane - Polk Hobbies Dorothy II

http://216.86.38.243/POLKS/itemdesc.asp?CartId={9AA4C151-E39F-42EVERESTF8-8504-C7BA2B737C77}&ic=PLK10045&eq=&Tp=

Cheapest Engine - Super Tigre GS-40

http://www2.towerhobbies.com/cgi-bin...?&I=LXFV69&P=0

Cheapest 4-channel radio system - Hitec Laser 4 w/ flight pack

http://www2.towerhobbies.com/cgi-bin...I=LXANT6**&P=0

This will give you a high quality airframe, motor, and radio system for around $218 plus S&H

You will need to add a propeller, spinner, glow fuel, and glow starter to be ready to fly. This would be a nice, high quality r/c setup for very little investment. You would save yourself a bit of construction time if you spent a little extra and got the Tower Hobbies Tower Trainer .40 MkII RTF package:

http://www2.towerhobbies.com/cgi-bin...I=LXCXF4**&P=0

Only you know whether time is more valuable than money to you, so only you will know which is the better option. Good luck.