IMHO steel is a acceptable material for making a robot drive train. I like it because it will keep the weight down low to improve your COG. Steel is appx 3 times heavier than alum but designed correctly can be much stronger/ less costly in the same size package.
Steel is much cheaper in terms of cost/lbs vs alum. CRS and stainless sheet is readily available from material suppliers. Steel can be easily spotwelded and mig welded. Welding steel is much easier than welding alum. Laser cutting light guage steel is significantly faster than cutting the same thickness aluminum.
Here's some stats you can use for you analysis
Cost/lbs of Cold roll sheet sheet is appx. .55/lbs
Cost of Alum 5052 is appx 2.00/lbs
density of steel is .29 lbs/sqft alum .098 for alum.
Take a look at using .036 or .048 CRS for your sheet stock and using .065" thin wall 2x1 , 1x1 tube.
Here's a website of a company that can laser cut tubes. Think of designing tab and notches to click together your frame and use the sheet metal as gussets and brackets
http://www.tubeservice.com/index.html