Everyone seems to have a little different idea about what "commercial rights" means, and how high/low they should be priced. In my own case, I have a personal hard and fast rule that commercial rights on my gigs will never exceed the starting price of the gig. It's annoying when, as a seller, I follow up earnestly with a seller and see that the starting rate is just a kind of a "bait and switch" trick. It strikes me as unscrupulous and sneaky (whether it is or not). But I digress.
As for the threshold, I like the simplest one I've heard used. "If this is not for your own, personal use, commercial rights will apply."
Whatever you set as a policy for which situations require commercial rights, do yourself a favor and be consistent in enforcing it. If circumstances lead you to conclude you need to make an exception, then make it very clear that it is that...an exception. The reason is that commercial rights can be a good accumulative source of additional income. Giving in all the time, or never even making an effort to follow up when a buyer fails to include them, to coin an old gambler's phrase, "leaves a lot of money on the table."
Today, I sent a buyer a request for the CR extra, and they politely said, "We haven't decided whether we're going to use your voice on this for sure yet, but if we do, we'll add commercial rights." If I'm to believe what they are saying, they were paying me to try out my voice, essentially a paid audition. I thought about it a little, and decided I'd give them a break, but only a one-time break, since after this, they'll know what I sound like. I wrote:
"I'll waive it this time for you. On future orders, commercial rights will always be required, since this is being recorded for a commercial concern. Thanks."
That will please them, and also make it clear that in future, they'll need to include it.
While we're on the subject of enforcing commercial rights, here's a word hack that has worked great for me:
"Here is the required commercial rights extra."
Including the word "required" puts a stamp of finality on it, and has resulted in nearly all buyers accepting it without complaint. But part of the reason they accept is probably also due to the fact that I don't have my gig priced "upside down." My commercial rights never cost more than the starting price of the gig. So, it feels less injurious when I require it.
One more thing you can do to be "the enforcer" when it comes to commercial rights, and I think this one really helps the cause. In my gig description, I have the following:
"IMPORTANT: Be sure to add Commercial or Broadcast Rights when required, otherwise, files are delivered "watermarked."
By that, I mean that, in the case of audio, a tone is superimposed over the top, to prevent it from having any practical use. On several occasions, I have reminded people in my messages as well, when a second reminder is necessary, as follows: