Picking up commons or even uncommons in bulk is a great way to go - as long as the seller stipulates the policy on duplicates.
The going rate for the singles at the local shop here starts at $2, so Auggies (as recommended above), with some commons at $.50, with $3.50 shipping, looks like a great way to go. Of course international shipping will have to be taken into consideration before you would consider it a deal or not, right? They even bundle one common in groups of tens for even lower cost per mini.
If you go the booster route, it is a great way to get some rares you'd likely not get otherwise, and get some extras that you can trade (be sure to keep them in the bag though). DDMs have a knack for making some of their most iconic figures, such as the beholder, demons or dragons as rares. Just remember, for every one coatul you'll likely end up with three dire bears!
It is really cool to field a horde of orcs or kobolds with minis, and I've found using other common minis, like Warhammer, works as good. I just look at the price vs. what I really want as the most important indicator for whatever I purchase.