I don't know what you picture when you think of a workbench, but I personally hate wobbly or lightweight work surfaces, so I have a tendency to over-engineer my workbenches.
I made the top surface by assembling a number of two-by-threes.
Each one is drilled with 1/2 inch holes at the middle and ends, and then threaded rods pass through all the boards to tie things together. I used Liquid Nails between the boards.
I capped off the ends with some more 2x3, screwed and glued.
I then whipped up a set of legs using 4x4's and 2x3's.
Next I assembled the pieces, screwed down some 3/4 inch plywood and stained/sealed the whole bit with many layers of poly.
A few finishing touches likea some metal drawers from an old desk, plus built-in 110v outlets underneath, and I think it turned out pretty good.
Now, I didn't plan this thing with the intention of putting the entire bike on top of it, but I wouldn't hesitate to do so ,as it's certainly sturdy enough to hold that much weight. I also never fear breaking out the hammer, this thing has enough mass that even my biggest hammer barely makes it shake.
Total cost was around $50-$60 I think for the wood, stain, poly, screws, and Liquid Nails.
cheers
mystic_1