Installer is charging $300 to install two hummingbirds, duvets, on ram mounts, and an off switch on the console.
I supplied all parts.

Is that an average cost?

Sounds high to me. Figuring at around $80/hr that's 3.75 hrs. Not sure what kind of boat you have and difficulty in running wires. I did mine myself and no took no where near 4 hours. Best to get some estimates from a couple more places.

I would charge at a minimum $300, to install them right takes time.

$300 for a tech to spend a half a day working on anything is actually pretty cheap IMO.

Prep work, labor, misc parts and pieces that might be needed, and clean up. Then there's shop overhead and possible warranty work (figured into everything). Plus, and this is definitely a factor, the knowledge to actually be capable of doing the work right has to be worth something.

I personally wouldn't spend that since I'm a DIY guy, but I don't see it as being an unfair price. I know I charge more than $300 for a half day of work in my industry. I only make a fraction of that since all of the above is a factor, but it's what I have to charge to stay in business and have a paycheck on Friday. If you aren't making money, you don't have a job, you have a hobby.

I am at $349.00 for installing a bow and console units but your units will be wired directly to your cranking battery using 14 gauge marine grade wire which has an insulation harness to help reduce any electrical interference. It also has the 3 amp. inline fuse holder and the battery loops for positive and negative post. I supply all materials.

You also get a on hands system review on how to work your new units.