The price per fixture is situational.
A chunck of my clients live in single family dwellings built prior to 90ºC wire. I am almost always working directly for the homeowner, so communication is direct. The wiring method at the fixture runs the gamit from most common as #1:
2. BX (pre-drain wire, rubber insulated cloth covered conductors)
3. Flex (T or TW)
4. Rigid (rubber insulated cloth covered conductors)
5. NM and NMB (the innercity "all-metal-code" relaxed and allowed NM in 1975)
The first conversation, the estimate, will bring out the changes of fixtures that the homeowner intends. I'll give my client the information about the importance of not overheating the wires, and let them marry that info with their aesthetics. The worst problem occurs when they have already purchased the fixtures, and they are on shoe string budgets. Like as not, these folks will not hire me, or will exchange the fixtures.
I'll be at most of the jobs a couple days, so, when there is the need to know the price ahead of time, I'll pull the old fixtures first, put up pigtails for temp light, take stock of the visible j-box wiring. At that point, I'm comfortable with giving a price, explaining that buried violations will add yet more. In my experience, the visible wiring in open mechanical areas is a great indicator of what I will find inside the wall, and will guide me in my cautions ahead-of-time to the homeowner.
My take on the 90ºC fixtures is that they will heat the supply wiring and all the other conductors that may be in the j-box. All the wires that terminate in or pass through the fixture j-box must be 90ºC.