I get what you're saying. Basically say your truck needs a trans fluid/filter change and diff fluid swapped out, it should be a fixed rate since it's simple, right?
But if you need an engine done its more labor intensive and needs more skills/tools so obviously it requires more money, right?
Thats kind of how we do it, we have a "base" price for stuff like brake jobs, fluid, filters, etc. then it goes up and down based on labor times or part prices.
Like brakes start at $100/axle i think, and go up from there depending on if we can cut the rotors or if you need new ones. Same with trans fluid, if we can just drain the fluid without dropping the pan i want to say it's like $100 including fluid, unless it needs like 8 qts (most are 2-4qts), then it goes up obviously, or if we have to drop the pan.
Clearly engines are a bitch to do, I wrapped one up on a 2008 Ridgeline with close to 300k today, it wasn't easy, but it was a $7500 job. He also got a killer warranty on it (3 yr, unlimited miles) for like $1500 in labor (18ish hours) which basically included reman/renewed everything on the engine aside from accessories.