Hi,
You will unfortunately get a wide range of pricing. Some potential drivers: Quality of work, travel distance, experience level and specifics of the job (how difficult to pull wiring, flush mount or stationary mount, location of display and transducer (access, ease of install). That being factored, there is the need factor - do they want/need the work. If loaded up, some will toss a big number - if you bite, they take it because it's a big pay day. If not, they will let you walk.
I usually buy my stuff and have the dealer install. I negotiated this as part of the deal at the end after we settled on the boat. I've never done it post sale and I know I got a very good price.
I'm going to go out on a limb on this one. I'd say 25% of the equipment cost (assuming a sufficient amount of stuff like radio, plotter(s), ducer, radar etc. Maybe for a small job it is 35-40%.
What do folks think?