The Part 135 Charter Market is not an easy market to just apply (paperwork) and start flying for compensation.  Usually the paperwork route takes an enormous amount of time and money to start from scratch to actually day one of flying.  As stated from people who have gone the route, is that they have waited two years just to get approval.  Reason being the FAA FSDO won't look at their paperwork until at least 6 months up to a year due to the backlog of applications.  Then there is the process of having inaccurate paperwork or paperwork not being current after the FAA starts to look, that will delay the process even more.
 
The established owners have went through the process in starting their company which makes the prices what they are.  They have kept an accident-free lively-hood in which safety is their main goal.  Now most of these companies are not just a start-up business which even adds to the years of staying out of the "Hairs of the FAA".  This is a reason to sell for the price which it is currently.
 
The price range is what the market will hold now.  Not saying that you might find cheaper companies, because they are out there.  But with the price attached you would have to worry about the past history and future litigations pending, which may be the reason for the prices being so low.  
 
The workload dealing with an established Part 135 is an easier process in that you can just add your aircraft (or theirs) and start operating for revenue almost immediately after taking a check-ride with the FAA.

                                                                                                                                           Thank You - Joshua