First find out what's wrong with it. Your mechanic was right and conscientious to inspect the car first and to not take your money when he knows he can't provide the proper service, but unless something is coming apart inside the rack assembly, they do not need to be replaced. If something is coming apart, you should have noticed some symptoms such as fluid leakage, noise, binding, or a clunk.
Rubber mounting bushings can deteriorate and allow the rack to move sideways. That will tire you out from the constant steering corrections to keep the car going straight. Those bushings are often not available separately. The inner tie rod ends can become sloppy too. Those are a part of the "toe" adjustment. That's the final alignment adjustment that sets each wheel to nearly straight ahead when the steering wheel is held perfectly straight. A very small amount of sloppiness in any of the tie rod ends will allow the toe to change significantly resulting in tire wear, the inability to adjust it during an alignment, and if it's bad enough, constant steering correction on the highway.
Worn inner and outer tie rod ends must be replaced before doing an alignment. The inner ones are a part of the rack and pinion assembly, and they usually come already assembled together, but for almost all cars, new inner tie rod ends can be purchased separately. The exception is when the car is still under warranty. Then the manufacturer supplies the new rack assembly with the tie rod ends already on it. In fact, many dealers parts departments can't even get just the inner tie rod ends from the manufacturer. They have to buy them from the local auto parts stores.
As for cost, we don't get involved with that here because there's way too many variables, but it does seem awfully high. You can't go by what you find the part available for either. Different rebuilders do different things that affect the quality and cost. The shop will mark up the cost just like at Walmart and any other store. That partially helps cover the cost of doing the job over a second time at no cost to you when a new parts is defective. That leaves a lot of dollars for labor. I can only suggest that when I left a very nice family-owned Chrysler dealership in '99, I was their suspension and alignment specialist, and replaced my share of rack and pinion assemblies. You only installed a new one if the vehicle was still in warranty, and those cost around $450.00. It took as little as 20 minutes to replace them on some vehicles, plus the alignment. The average time was just over an hour, and one really miserable model took about 3 1/2 hours. Even at the $100.00 per hour shops need to charge today, that would imply it's going to take all day to do the work. Your car is not easy to work on, but I doubt it should take all day.
If you don't know exactly what's wrong with your rack and pinion assembly, have the car inspected at a different shop. Go to a tire and alignment specialty shop, but don't tell them right up front about the rack and pinion. If there's a problem, they should find it. If they are told about what someone else found, they might look extra hard to find the same things which might not be something that needs finding.
Once the alignment is done, you should find the steering wheel is straight when you drive on a straight and level road, the car should not pull to one side when you let go of the steering wheel, and over time the tires should wear smoothly and evenly across the tread.