The other thing I might add is that on the 'per inquiry' end, the toll free number given is exclusive to the market. In other words the number you hear is for that specific market based on the zip codes the station reaches via on air and cable reach. It might be the same number in Milwaukee as Dallas or Las Vegas but no two stations in the same geographic area have the same number. They credit the station based on the zip code of the caller.
Most of those spots on TV with the toll free numbers offering a product (usually under $20) is a per inquiry spot.
Every radio station I've worked never got a decent check from a per inquiry deal. Most were around $20 a month and a few would reach $75 or so but once we cracked $100. Given the number of airings, it was pennies per spot.
On the radio end, we always went cash in advance on infomercials. I had a couple that were poor sellers and I had to give them a few bonus airings but usually it worked for the agency buying and we got repeat business. One selling vitamins actually bought 5 hour long infomercials a week for about 3 months. Theirs worked likely because it sounded like a live talk show with callers and giving out a toll free number for asking the host a question on the show. In fact, we pulled it off satellite back then...different show/callers each day. Given our lack of ratings and how hard we worked to get billing, that was pretty sweet.