People have memories like elephants for negatives. While most any recording studio can record a VO demo, most don't know how to produce them. However, if you have a friend at a radio station, you might want to check there. If you work at a radio or TV station, you'll get off cheap. The cost will depend on where you go.
Some studios charge a flat rate while others have an hourly fee.
If you have existing spots and it's just a matter of editing, the cost will be less. Not only will you have to pay for the studio time, but you'll also have music selection, which can take a while if you want the best possible music for your spots. And that's only for your master demo reel. You have to have the reel duplicated. If you're moaning over your checkbook at this point, be reminded that it takes money to make money.