With shoes, like anything, you get what you pay for. If you've got cheap shoes with soft soles that you wear every day (and you never rotate your pairs around), then it's not unreasonable to wear them out in a month or two. Heck, even fairly pricey shoes will wear out quickly if they're the only pair that you wear--I have a friend who spends lots on high end Tevas and since they are the only shoes he wears, he gets them replaced every six months.

I'm not sure what you mean about wearing out under the heel--do you mean on the outside edge near the heel? That's overpronation. If it's the inside edge, then you underpronate. Either one of these can be helped with inserts. If it's just the heel in general, it's a combination of your stride and probably the material of the shoes, in that if your stride makes you repeatedly walk on a relatively weak part of the shoe it'll wear out. Or, it could be activity related (if you ride a bike a lot, you could be dragging that spot and not notice).

I overpronate myself, and I used to need street shoes for my job that are similar to what you wear. I had good luck with Sketchers and Docs, in that a $90 pair of shoes (in rotation with another pair) would last several years. And, since they were relatively well-made, I was able to get the soles repaired when my gait ruined them.

Whatever you do (if you're content with cheap shoes you replace frequently, or want to invest in better shoes that will last longer), get two pairs and rotate wearing them, i.e. pair 1 on MWF, pair 2 on TTH. This both spreads out the wear and allows the shoes to dry thoroughly in between wearings, which helps with avoiding stinky shoes and some foot fungi.