That's an amazing deal. You'll find the prices vary quite a bit from state to state, but it's not unreasonable to pay upwards of $40k for a kitchen remodel. $28k is still a very good price, depending on the quality of the cabinets. Super-nice and top-of-the-line is more like $50k+ (to start).

BTW, cabinet doors are what really determine the quality of most products. The bottoms and sides are generally all particle board. The next big cost factor is the quality of the veneer and whether or not it's real wood. This will generally only be important on exposed cabinet sides (gables). In some kitchen layouts you may not have any at all.

A plumber in these parts is likely not to bother with a such a sub-contracted job unless he knows he can come out and make over $1000 for the day. I don't know what the $4000 quote was for, but it may not be out of line for your area for professional plumber service.

Lowes, Home Depot and Ikea all carry decent cabinets.

If you don't have prior experience dealing with sub-contractors, and want to act as the general contractor yourself for this job, it would be wise to source as few subs as possible so managing them doesn't get out of control. Scheduling is going to be extremely important since one trade's work can affect the ability for the other(s) to start/complete theirs. This is where experience dealing with specific trades and knowledge of building practices comes in very valuable.

This is what my brother does for a living, though he prefers to do most of the work himself when possible - since he doesn't have a large network of subs/workers he can trust and count on.