This not the cheapest furnace but i had to buy a lot of my supplies. If you are a good scavenger/bargain hunter then your total price will be less than mine. Also propane compared to other fuels isn't the cheapest either, but it is clean and efficient.
Warning/Disclaimer: Working with molten metal is extremely dangerous. I do not claim any responsibly for any injuries or accidents that may occur during replication of this instructable. Always be safe when working with high temperatures and molten metal (don't do anything stupid and always wear safety equipment).
Do plenty of research and know what your getting yourself into.
http://www.youtube.com/watch?v=lX6yFQNnk-A (this a charcoal foundry but there is still much to learn from it)