Prior to flying for my company I was a dispatcher/salesman. We did all kinds of quotes, mostly for freight trips and I, for the most part, quoted out our Learjet fleet. We did however do air ambulance and passenger quotes, both very different from freight. Air ambulance was a totally different animal so I won't really touch on that. As far as passenger flights I will give a little insight from what I have seen from being both a salesman and pilot of these trips. We could launch an aircraft in as little as an hour from the word go on the flight, anytime night or day.
Typically passenger flights are for business type deals, not so much pleasure flying in my experience. I think that there is a bit of a "status" thing involved on both ends when chartering a jet. One side is trying to impress by using a charter service and the other side is saying we have the skills and power to demand that we are flown somewhere in a private jet. Another important fact is that time is money. We went to a lot of out of the way places that were sometimes a 2-3 hour drive from the nearest airport with regular airline service. These little towns may have had a very important auto part supplier factory that needed a visit from someone to fix problems or from investors. A problem at that plant could shut down an assembly line on the other side of the country or world and could cost millions of dollars so dropping say $10,000 on a charter flight to fix something is nothing in my opinion.
We did have pleasure type trips though. Sometimes it would be a guys or gals weekend away to Vegas or Atlantic City or even golfing in Florida. Back when jet fuel was cheap, it wasn't that bad if you chartered a jet and split it 7 ways with your friends. It would basically be the cost of each guy buying a first class ticket if they could fill the jet. They could make their own schedule and again, fly into small airports and not deal with long drives and delays. We always did quotes by the mile flown, then we would tack on basic catering fees(if they ordered anything specific that we didn't stock we would charge that too), taxes, any parking fees and a daily rate if we were waiting. This has changed a lot with the popularity of fractional carriers here in the US after 9/11, passenger charters seems to be a little rarer. I don't know what our rates are specifically now but I would say they are at least double that of 10 years ago so I don't think it is as cost-effective for a small jet charter for fun. I have done some cool flights though!
It really comes down to how many people you want to move. For us it was the same price whether the plane had 1 person or 8 on board. On larger transports, I have no idea how it works, I think it's the same deal though, you are paying for the plane itself to operate and from what I have seen, they do it based on hourly rates. The best way to really figure it out is to see how many people have to go, get a quote, divide it up to see the per person price then compare it to an airline ticket price. I don't think chartering for a group of 10 or less is very cost effective these days because of fuel prices now.
I always tell people that do want to charter to contact the airline itself and do the legwork. When you call a broker, which there a ton of here in the US, you are just calling someone that is going to add a few percent commission onto your flights just for calling a couple carriers themselves. Heck, if you want to know what it would cost specifically, I would call a carrier and ask for a quote, I've never told someone no and it only takes a minute usually to work something up if it's simple!
Too lazy to work, too scared to steal!