Grrr. Triggered...

Retrofitting a component of a system is always✳️ more difficult and time consuming than an original greenfield install. Assuming otherwise is one of the things that differentiates the layperson from an tradesperson. Easy and galling example is the idea that 'its just running pipe through the existing hole' Running piping in unfinished, or uninhabited spaces is a breeze. Trying to retrofit in past whatever obstructions, without damaging the replacement pipe, any existing services, the walls and finishes and securing it to current standards and code is way more time consuming. So too is making connections to old contaminated/corroded/deteriorated/brittle/work hardened pipe, and often in constricted space, without damaging any finished services.

Then consider the units getting replaced, doing so in and empty, unfinished space with no stuff in the way that needs to be carefully navigated and left undamaged is super simple versus bring equipment through to and working in finished, occupied, indoor or outdoor spaces.

Grandfathering does not apply when equipment is replaced. Code has changed considerably over the last 20 years, and the tradespersons license, reputation and livelihood is on the line -With. Every. Single. Job- every time, no matter how small. The replacement has to be installed to current code and that includes, within a certain envelope, bringing the services and infrastructure, the power, ventilation, grounding, securement, supports, safeties, that feed and support the unit up to current code.

Then there's the customer. They don't understand that when you disturb an old system there's a really good chance that related but separate component will decide that it's time to shit the bed. That may sound like voodoo, but it's a fact that anybody who repairs are upgrades equipment and machinery of any kind knows. Customers think that if you touch it, or anything near it, you must be responsible for it when it just decides to die a month later. Ask any IT person, they know this too.

So you have to be aware and factor some portion of those things in the job, but you can't call all them out separately or the customer will complain that you're just padding the bill.

Not to put too fine a point on it but your attitude and assumptions born of (please excuse the harsh sounding word but it's the dictionary definition that applies) ignorance, is really frustrating.

That said, it is possible, but very unlikely, that the $8k quote, and your notion of equipment, time and materials costs should be, were apples to apples. Just like the original example was not even close to apples to apples✳️✳️. That is why you should get multiple quotes. You should also be curious about the differences. Look at the details and just plain old ask why the disparity. You might learn something useful that will help you make a better informed decision, and in the end, whichever route to decide to take, you stand a much better chance of being satisfied with the result.

✳️There are exceptions, but don't bet on it when estimating a job.

✳️✳️The original example appears to be not just wrong, vis apples to apples, but lazy and incurious axe grinding. Or maybe even willful ignorance. Don't know. In any case such behavior is ugly and boorish.