It may seem like it's expensive, but I don't think it's a ripoff. It's hard to know everything that's involved since you don't provide details. It sounds to me like the steps for the plumber would be: shut off the water (are there shutoffs under the sink?); disconnect the water supplies; disconnect the garbage disposal; disconnect the drain and pull out the old sink; pull out the old faucet; mount the new sink and caulk it; reconnect the drain; reconnect the garbage disposal; install the new faucet and connect up the water. Turn on the water and check for leaks. That's all likely to take a couple of hours if there are no major problems, like piping readjustments, fit of the new sink, etc.

If you're unable to do these things yourself, and don't want to pay to have this done, you should survey your friends and see if any of them have the skills and time.

Around here there's typically a minimum call charge of $150, you'll not go below that. The amount to do what you want is variable depending on what Kudzu says. If the existing sink comes out easily and the new sink goes back in and everything lines up, sure it can be done in an hour.

I've spent an hour just trying to get an old corroded faucet out (and to take the sawzall to the blasted thing because the nuts were just too far gone to grab on to.

I agree with Kudzu9 and would emphasize that one of the more difficult factors of the change out is having to reroute the piping,drains etc to match the new faucet set and sink. There is a high likelihood that there will have to be some adjustment to make it all fit. Then there is the question of the sprayer...are you keeping a side sprayer or changing to a pull down with the new faucet? That will also cause the piping to change. As much as I hate to side with HD on any thing, the pros that they contract with to do the work or usually tried and true and you always have HD to fall back on of anything goes wrong.
Id go with their bid and be thankful that you don't have to do it yourself.

That seems a fair quote.

I'm a handyman, and it seems that plumbing work can be one of the most underestimated jobs out there. It's all too common to have addl. things to do in order to finish "simple jobs", and only a fair amount of homeowner DIY hand-wringing can cure folks of that thought process.

When I forget the value of plumbers and plumbing work (eg. quoting too low to friends), I get a quick slap-in-the-face reminder. Last week it took me a couple hours just to remove an old 3 piece lavatory faucet set that was seized up and corroded. First, top was being saved and couldn't be removed. So I had the penetrating oil out (no luck), then drill (to drill and break retaining nuts that couldn't be reached - and afterward sliced my fingertip open with a shard of metal from the nut), then sawzall (used two blades) to cut off the loosed fixtures from above. Lot's of fun.

the old brass nuts can be very worriesome..
i bought the smallest and thinest dremel tool i could fing
and use it to split the nuts..or just cut a deep groove in the nut and then using 12" long big bladed screwdriver
put the blade in cut groove and turn..like you are unscrewing it...this "pops" the nut open
or...cut that sucker with a chisel..
if i cant remove the sink, cuase thats the easy way