The cost drivers, in order of impact
Panel capacity comes first. If your service has room and an open double-pole slot, you avoid the largest single expense in the project. If a service upgrade is required, it can exceed the cost of the charger installation itself.
Distance is second. Every additional foot of conduit, wire, and labor adds up, and runs through finished ceilings or across a crawlspace cost more than a run along an unfinished garage wall.
Third is the installation environment: outdoor mounting, trenching to a detached garage, or working in a tight mechanical room all add labor.
Fourth is the hardware itself, plus permit and inspection fees, which vary by jurisdiction.
Why quotes differ so much between installers
Two estimates for the same house can differ because one assumed a simple garage-wall run and the other planned for load management and a new subpanel. They are not quoting the same job.
Sending the same photos to every installer is the simplest way to get comparable numbers.
Ways to keep the cost down
Put the charger as close to the panel as your parking layout allows. Consider a 32-amp charger instead of 48-amp if your daily mileage is modest. Ask whether load management can avoid a service upgrade.
And ask whether the quote includes the permit, the inspection, and the hardware, so you are comparing complete numbers.