There are a wide variety of off-the-shelf insoles available for running shoes. There are some better then others, of course. There are some that are not supportive enough and others that claim to be more than they are and are as expensive as a custom orthotic.
The typical drug store insoles, such as Dr. Scholl's, are too flexible to provide you any amount of control when you are running. They will offer some cushioning but don't rely on them of you are looking for support.
On the other side of the coin you'll find expensive hard rubber, leather, or plastic insoles. These are often sold at specialty stores such as Ideal Feet and Foot Solutions for $250 or more. Ironically these are similar to insoles sold on late night infomercials for $29.99! Be careful, once you walk out of the store they will not accept them for returns.
You should expect to pay $30-$60 for an off-the-shelf insole. You can find these at athletic shoe stores and running stores with brands such as Superfeet, Spenco, and SofSole. We conveniently offer Powerstep insoles at our Houston podiatry office as well as on our online store.