The supplies are cheap, relatively. if it is 3 hours of work, and its one person, thats $300 an hour, allowing $75 for a bit of lumber and sundry materials.

My cabinet install guy charges $275 for a sink install as part of the additional services available during a cabinet install. Add in the plumbing hookup for the sink $100 (with new shutoffs and lines), and a DW install ($150) plus the materials to do the cradle and any needed for the plumbing portion, and he'd be around $625. That's in a very low labor cost market. In a higher labor cost market, like NYC, I can see it being an additional $300.

Of course, this all depends on who is doing the counter cutout fabrication for the sink. Most counter fabricators install the actual sink as part of the initial counter fabrication contract in order to be sure that the sink fits correctly. Then a plumber provides the plumbing connections and DW install. If he's doing the sink cutout (and polishing it correctly) then that's a pretty good price---IF he realy has the skills and tools needed to do the job. Like I said, this is usually part of the counter fabricator's job, so I'd be kinda skeptical that anyone else would really know what they're doing here.

The company I work for charges $350 to hook up the sink, DW, faucet, & disposal. Add another $100 in labor and $20 in parts for the framing and you are around $470. Seems a little high to me.

You didn't say where you were. Location figures in big time in labor costs. It doesn't sound unreasonable to me where I am living where you get things like $4000 to replace an electrical box.

I shoulda been a plumber rather then a teacher.

I actually know a dentist that had gone to a VoTech HS and did plumbing in college and dental school. He actually went back to plumbing after a couple years in practice because he made more money as a plumber than as a Dr. This is not always the case but his perfectionism kept him from making a lot as a Dr., but kept him in demand as a plumber.

I'm not sure what costs are relative to your area but maybe you could get another quote? I have done a number of projects and on the first I got multiple quotes to find who I wanted to work with and I've simply stuck with those people for the future. So, you don't necessary need to go through the multiple quote game every time but here it sounds like you might be unsure so the effort might make you more comfortable.

Also, I've found that people who are padding their bids tend to get flexible pretty quickly when they find you are getting other quotes :)

Many people love to overkill and overbuild. When they specify what needs to be done, they go too far, demanding things that are unnecessarily strong. There is no end to this.

Internet strangers won't be able to guide you well. How much you can eliminate isn't something that is ever easy to explain. Writing a bunch of words may cause a lot of anxiety in other readers who then might chime in with more words ....

Basically, the quote may have been for a lot of overkill, and for this you might share responsibility. Calling it "... build a custom wood frame into my existing cabinets to support it ..." is what makes me think this. On the other hand, if I try to describe how simple the wood support bars could be, I find I end up deleting my words often, because I have seen many other situations where people pile on when I write in my own words how simple something can be. But, I'll give it a try. It's just wood sticks. I think that someone who knows what is strong enough, in person, on site, can judge what number of screws and what size wood bars will do. The bars don't need to be "a frame". They are bars screwed onto another wood. Some of the screws have to reach the cabinet's side panels. Many of them. The longest bars are a pair parallel to the long side of the sink. These have to be long enough to reach the side panels on the the left and the right hand side. That is necessary to ensure that nobody is screwing them to a cabinet's wood top (which is not rigid enough, over time, as it sags). Once you have bars crossing the space as I described, you have achieved 90% of what you need to do. (( " 90% " is used figuratively to mean a lot ). An argument can be made that this is basically all you need to do. That argument has validity, but it can be refuted too, with counterarguments. So, to avoid a never-ending discussion that leads to sleepless nights, just build a little more strength onto the two parallel bars. HOW you do this is up to you.

Hth

In my case, I didn't use wood. I went with steel bars. Angle iron.