Bottom to top!
Consultants in most professions get paid a much higher daily rate than full time employees for obvious reasons, but not for a TRE/TRI/SFI.
A TRI with an established company may earn 130k, and with all benefits and costs will cost the airline 170k. If lucky the company will get 180 days work for that and many of those days will not be training days. It is cheaper to have a core training department and pay for consultants, through an agency, as and when needed. The airline saves money, the agency gets a good cut but the agency trainer ends up with only two thirds of what should be the going rate, at best.
The agency pays the minimum it can get away with. Just under £500 per session with one agency I believe and less with many others. By offering Trainer courses the agencies then have a ready supply of trainers in addition to those topping up pensions or already trying to scrape a living from consultancy work. Many of the self selected trainers will be good but many others will show why they were not selected for training positions by their airlines.
The overall effect is that career progression within some airlines is curtailed: More and more pilots who would not normally be selected for training positions are training: Training itself becomes more by rote with the cadet who best learns the company chants and mnemonics being preferred to the one who can actually fly and the art of proper instruction being replaced by 'cheat sheet' training. ("Here is the script - just follow that.")
No blame can be attached to anyone who takes advantage of the system allowed by the regulators. (It is possible to do a Type Rating course and not see an instructor who has actually flown the Type until the final simulator check!) Only a dearth of applicants for bottom and top jobs, MPL and TRI/SFI, will change the terms and conditions. I know of many excellent recently retired training captains who would be welcomed by many training organisations but will not work for the low renumeration offered. Just like experienced first officers, they are priced out of the market.