If the service were free would you accept it?
If not free, just how much do you think something like that is worth? How much is an educated, perhaps professional person's time worth. You are paying for more than just a volunteer docent at your local flower park on once a month 'open houses'.
The guide (the company) has to earn a living. $400 for a single person for 7 days might not sound that bad. That's about $60 a day. It is not a vacation for the guide, so he is allowed some compensation. It is a long (24 hour) day and a lot of labor, but it is personal service and that guide provides at least for himself and maybe others at home. Not sure what the guide would be paid the other 7 days if he were not on the trail with you. He is also liable for your skin.
Six of you involves a bit more work for him, but maybe not directly scalable to $2400. But it is still a personal service. People costs are expensive, especially if they 'are yours' indentured for a period of time
In a week, you can only see so much different geology in the Sierra - walking. There are only a limited number of flowers (30-40 in bloom when you are there would be amazing), and some shrubs and a few trees. Toss in a few fungi, some history of the Indians (not a lot up high) and the prospectors (most went bust anyway) or early settlers and who constructed the trail, the 4 or 5 popular constellations (and a few WOWs) visible at night that week and you have the basis of what you might get out of a good guide.
A better plan might be to spend $200 on books, pass them around among the group this winter, before you leave. Each take a book with you and act as the expert on a subject. At the end of the trip, pay them each what you think their 'expertise' is worth. You will probably remember more if you start studying now.
OH! And don't forget the substantial tip (which is getting to be almost mandatory on guided tours).
Let's say you got to Europe on your own hook - not a 'tour', but you want to get as much information on your own, picking and choosing as much as you can in your short stay. If you were to take the multiple tours per day in, say, Germany, on your own, to get the information you would not know about the churches, museums, and city in general, you could be paying almost $100 a day (over and above your living costs). You would probably be in larger groups -- the kind with the leader running ahead waving his brightly covered umbrella in the air to keep you well herded and not too lost. You would be buffeted by others in the group trying to get up to the front of the crowd to hear what was going on. Many times you would end up getting to the vista point in just enough time to hear, "Any questions? Right. Moving on". The tours DO get you to the front of the lines at the popular sights.
Have you tried to haggle the price down to something you might agree as reasonable? If not, enjoy the trip just like a bazillion others before you have and make a determination, afterward, if a guide would have enhanced the time you spent by THAT much more.
There are a lot of reasons why you are going to enjoy the heck out of this trip - with or without a guide.