Well a lot has changed in EMS over the last 10-20 years that increased the cost, but a lot of it was for the better. Look at the equipment we are flying now. There are a lot of twin SPIFR aircraft. Even the singles are 407's, ASTARS, EC130s. (Almost) no one flys 206s anymore. These twins and bigger singles are way more expensive to buy and maintain. Training has gotten better, more advanced, and more expensive. Pilot and Medcrew pay has increased. Something no one thinks about is the cost of a positive safety culture. How much does each WX turn down cost when it might have been possible to scud run it. This is a good change, but it comes at a cost. WX minimums and repercussions were different back then.
Of course a lot of the increase in cost is due to flight volume decreasing. With so many bases being added everywhere, 25 flights a month has become the norm, instead of 60+. The fixed costs remain the same, aircraft, hangar, pilot/crew salaries. Less flights mean less payers to divide that fixed cost. Combine that with poor reimbursement rates and now there are even less payers to split up those fixed costs.
It's not just ambulance rates increasing. All hospital costs have increased exponentially.