A bunch of creative people have been trying different things for many years. At this point, I haven't heard of anything more cost effective than wood. George Dyson (and many others) have made extensive use of extruded aluminum tubing for frame materials, and it works very well, but does cost a bit more than wood.

If cost is the driving limiter, scarf wood together from whatever clear or nearly clear pieces you can scrounge. Use exterior paint leftovers from somebody's garage. Borrow tools. A good scrounger can be on the water for less than 50 bucks.

If you are just looking for creative new ideas, search the archives of this list, the archives of the baidarka list, and the qayaqusa list, (any others?) and then get busy. We all want to hear about it!

(I'd shy away from the ferrous metals, though.)