You are paying for three things: the diagnosis, the part, and the labor to put it in and set it up properly. Setting it up properly means balance, force settings and a full-cycle test, not a part dropped in while the door is left exactly as rough as it was before. That last bit is where a lot of cheap repairs fall down, and it is the part you notice six months later when the opener is straining again.
We do not post a flat price list, and there is an honest reason for that. Spring size, door weight and opener model all change the job, so a number quoted before anyone has seen the door is a number we would just be making up. We would rather look at it and tell you the real figure than win the call with a lowball we cannot stand behind.