The thing is, where lenses are concerned, improvement in performance does not scale linearly with cost. A lens that is 10% better optically can cost 2 to 10 times as much. Especially fast aperture lenses.
A one-stop increase in aperture (eg f4 to f2.8) means theoritically a minimum of double the surface area, and practically a lot more than double the amount of glass, as well as a lot more aberrations to correct.
If you're new to photography, i'd suggest you go easy on the purchasing. But stay away from the bottom of the barrel lenses, those very cheap kit lenses and those super-zooms (28-200, 28-300). The lens you mentioned (28-105) is considered a good lens, but i think there are two versions: one starts at f3.5 (the good one), the other f4 (the, um, not-so-good one). Which one is yours?