Well Dave Lawrie and I had a similar goal back in 2006 to build a heavyweight for next to nothing.
Our mission was simple. How much where we willing to put in each to have a completely expendable heavy. We had 300 pounds in total and after using 80% of a decent wheel chair chassis, a speedo which was servos hitting micro switches and relays and finally a ton of ply wood. We had a heavy.

http://imageshack.us/photo/my-images...1lv0.jpg/?sa=0
http://imageshack.us/photo/my-images...hxp0.jpg/?sa=0

He fought once and survived but had a technical fault in the pits and died from mass rain at the outside event.

It wasn't fixed because we were so happy with the internals we turned it into cabraken with Dave Weston joining our strange heavyweight venture

cabraken.jpg


This was designed to be indestructible and just a fun robot to bring to events and fight for a bit of fun. Budget was a bit higher but not silly money. Less then 500 I think. Mainly had 25mm hdpe bulkheads. Steel angle, bolts and 6mm steel armour. SLA batterys and a wotty which I used in my feather.



However it wasnt indestructible and an aspect which I didn't consider was that you need to be prepared to replace every part which could fail. We lost a motor which wasn't replaceable as it was from a very specialized mobility wheel chair. Which meant we had to get new wheels and the robot didn't feel the same.

So I dont know if my adventures in cheap heavyweights will help but thought I would share it. Maybe I would suggest you grab a wheel chair and have a bit of fun but be prepared to lose it all. Dave Lawrie had good success solo by using pre built motors and gearboxes from robot market place.