Have an upcoming trip with a group of 6 where we will spend one night at an airport hotel (Campanile). The hotel shuttles charge $5 per person, as I recall, and you have to lug your stuff to the shuttle stop. Will regular taxis (we will need an espace, of course, but I have done this with 6 people) make this short trip for a comparable price? Seems much more convenient to me.

I don't know why not. I doubt they're allowed to refuse you, and since it's a short trip, they can be right back at the airport to pick up another fare in a jiffy.

A big taxi will certainly cost less than 5 euros per person. I am already appalled that the Campanile would even quote a price in dollars.

In any case, some taxis might refuse you for this short trip (even if legally they should not), but others will accept you, and since there are so many taxis waiting, it should not at all be a problem. The taxi attendant will find a taxi for you.

It is sort of a gamble for the taxis, because it is not at all true that the taxis can return to the airport for another customer "in a jiffy." They obviously have to go to the end of the line, and the line is generally about 90 minutes long, according to the electronic sign which is there exclusively for their information. However, the CDG Campanile is quite big, so there is a good chance that they can get another fare going somewhere else -- or they can drive back to Paris for their usual business.

The problem is that taxis can wait at CDG for an extended period of time in anticipation of a 50€/55€ fare into Paris. Understand that there are drivers that rent their taxis from an owner at 12€ per hour and who would very likely not be overly willing to lose money taking you to the Campanile.

There are other ways to look at this. One can stay, for example, at B&B Hôtel Paris Roissy CDG Aéroport, practically next to Campanille with free airport shuttle.

However, the official taxi dispatcher is there for the customers, not to please the taxi drivers. For example, if someone says "I am going to pay by credit card," numerous drivers might say "oh, my machine is out of order" but the dispatcher will find a driver who will say yes. More trivially, you have the right to choose any taxi in a taxi queue, so if you want to take a Mercedes, and the first Mercedes is #8 in line, you can absolutely get in it no matter what the other drivers say.

So frankly, I do not think that there will be any problem finding a taxi to go to the Campanile.

Just head to end of taxi queue won't lose place and take a low fare then?

A taxi is a taxi, they are supposed to go anywhere.

That's an interesting question about what happens if they refuse at the airport? I don't know. I have, in fact, had a taxi refuse to drive me about a half mile from a train station in Paris (due to it being too short), although the queue thing isn't quite as regulated there as I recall. So whether taxis are allowed to do certain things or not, in practice sometimes they do.