If you want it to last forever and survive a few scrapes, then a fairly heavy (3.5 lb) titanium frame is the best choice. Steel is probably the second choice, followed by aluminum and then carbon. Carbon frames can be light and strong, but if the yield point is ever exceeded (which can easily happen in a crash) then they are toast.

Here is a Ti frame that would fit your budget: http://www.habcycles.com/road.html