Like just about every other price in this world, the price paid for instructing is governed by market forces. When the supply of cheap instructors dries up, then the wages will rise to maintain the supply!
Originally the instructor rating was used as a route to get the necessary hours/experience element of a CPL. Hence there were plenty of fresh new PPL instructors around all busily hour building but not instructing too well because of this. Then they introduced the BCPL in the hope that holders of this would either go commercial or instruct. Not a bit of it - instructing was still a route to get the necessary experience. Today is the same. Commercial jobs are hard to come by, so many get their CPL, then do an instructor course so that they can build cheap (free) hours to make themselves more attractive to a commercial employer.
Making a profit as a flying school is hard enough as it is, so all costs must be minimised - including the cost of instructors!
On the helicopter side the situation is different. As a CPL(H) is not the start of a route to the airlines, there are fewer people around who hold the appropriate instructor ratings and the instructor fees are much more realistic.
Years ago I used to have fun comparing my hourly fee as a fixed wing instructor, BX examiner, ATPL holder e.t.c. with my colleagues in other recreations. I discovered that my hourly rate was less than a basic horse riding instructor, a music (piano) teacher a driving instructor, a scuba diving instructor, a golf pro, a microlite instructor and (incredibly) a model helicopter flying instructor!
As others have said, you will do well if you have a private income but I made it a hobby for 25 years and enjoyed trying to do a good job. The satisfaction is there but until the flying instructor rating is no longer a route to an airline position, the fees paid will continue to be cr*p
P.P.