That is a fine price point. Perfect for a used sigma 105mm. I would suggest against the tamron, it is not a true macro lens and does 1:2 magnification instead of 1:1. Your other option for about 400 is the Canon 100mm USM, or sigma 150mm, all great lenses. The disadvantage to the Sigma 105mm is that older versions do not focus internally, so the lens will physically move as it focuses, making tripod work more difficult. It is still one of my favorite lenses, not only does it take incredible tank shots, but it is a portrait all-star.
I switched to Olympus, but still have my sigma 180mm macro for Canon, it's wonderful, especially for getting deep into the tank. They are harder to find and a little more expensive, about $500. Unfortunately I sold the three lenses I recommended already, but they are fairly common.
I have tons of pictures that I took with my T2i and those four lenses, it is a wonderful starting camera.