Just replaced my old bench, it was tiny. It was 3 feet long and my new one is about 11 feet and only cost 12 bucks...
I used some old 2x4s and made supports for it and bough a 4x8 piece of 3/4" OSB and cut it up...

Laid it on my supports I made and viola... maybe took 3 hours, including tearing out the shelving that was there and my old work bench.
You might notice the mess over to the right, yea, I had to pull out the shelving all that stuff was on, now I gotta that out...

It doesn't look great but will be extremely functional and a lot better than my old one.
Here is a pic of my old bench while I was working on a cab...

In case anyone was thinking of doing this, but thought it would be expensive or difficult, its neither.