Understand that your commercial production is different than the actual time slots the client (the law firm) is purchasing. You produce one commercial for a fixed fee (Example), the client pays for their own ad broadcast X times a day times Y number of days (Example). Two different things.
True, although it's not uncommon that you would handle both, working in a sort of ad agency role. So you charge a flat fee for the production, then you make the ad buy and charge the client a 10-15% markup for the air time. Most clients aren't going to want to put in the time or deal with the sales people at the cable company, they'd rather just cut you a check and let you deal with it.
That also gives you an ongoing stream of revenue, and can offset lower production budgets up front.
Expect them to try to steal your client's (the law firm) business away from you.
Of course, that's the problem - just like you can offset the cost of production, so can the cable company, and they'll often do things like throw in free or almost no cost production in order to convince a client to buy more air time. The free commercial will probably be pretty bad - as so many local ones are - but the client often can't tell the difference or doesn't really care.
It's another argument for handling the buy yourself - you don't want their salespeople directly interfacing with your client, or they're likely to try and steal them. Handling both the production and ad buy yourself simplifies things quite a bit for the client, and if you don't do it they may well choose to go with the cable company directly.
One thing I've learned after years of working with local small businesses - marketing, promotion and advertising are generally a big hassle to them, and something they don't really understand. They just want to do whatever it is they do - be a lawyer, make coffee, sell fashion, etc. If you really want to succeed in that market you need to figure out a way to provided measurable results while taking the burden of worrying about it off them.
