Hi Michelle,
Another option, look for a lightly used 28-135 IS. It's an excellent lens optically, close focusing (and you can use macro extension tubes with any lens to get even closer focus), and decent build quality.
There are a ton of hardly used 28-135s around. They have been selling in kit with 40D/50D for some years and a lot of them end up as duplicates, so get sold off. It's not hard finding one for $300. I recently bought one with a lens hood (recommended, a $25 accessory normally) and a 72mm B+W MRC UV filter (around $75) for all of $275. It's like new, except no box. Normally the 28-135 sells for over $400. Serves as a backup for my 24-70/2.8.
This is a little more than the 28-105 costs, but I think well worth the difference for the Image Stabilization, if nothing else.
If interested, check the Classifieds here on the forum, your local Craigslist and used items of Amazon.com (that's where I bought mine).
I mentioned macro extension tubes so should clarify. There are really cheap ones ($50 and less) on certain auction sites. I don't recommend those. They are fully manual and a bit fiddly to shoot with. I think most people would be happier spending a little more to get the ones that have contacts so that both auto focus and the lens' aperture continue to work. Look at Adorama's ($100) or Kenko ($160). These are both sets of three tubes, which gives you a lot of flexibility shooting macro or near macro with practically any lens.
