Having formerly been the Production Manager for a large cable advertising department(I am now freelance with my own small, but full-service production company)here’s my 2 cents:
If you are using your own gear, just charge them your normal hourly shooting rate or work it out so that you can shoot multiple clients in one day and charge them a package price. Make sure that they provide or pay for tape stock!
If you are using their gear, they will try to get you for as little as possible, so make sure that you are charging them enough to cover costs. See if you can get them to pay you a decent hourly rate + mileage since in many cable markets the businesses can be spread out pretty far. It was very common to travel 100+ miles round-trip for 1 commercial! That will eat you alive if you’re not getting some compensation for mileage.
Make sure to work out payment details in advance(i.e. how long you are willing to wait to be paid). Most will be 30 days, but they like to keep their money in the bank, if you know what I mean. I did do some work and some equipment rental for my former employer and I believe the longest they ever went was 45 days.