There are many online resources, but typically they are subscription or fee based. I have not found a free resource or calculator with all the breakdowns. Typically I'd call my insurance man, my local appraiser or check my state website. I know Nevada put out a nice resource a couple years back on their website, we used it to help sister with her ranch.

I think to some degree that varies by region. I took an appraisal class that helped me greatly.

An appraiser may be able to help you, Danny.

Contractor, or you could go to a building materials place to price out materials.

Also the 2022 Cost v Value  might help.

Check with a local appraiser. Costs differ by area.

I would consult with 1 or more local appraisers. The values no doubt differ locally so the generic reports we see on line are likely off. I wouldn't base my professional opinion on them.

When needed, I consult with my local tried, true, and trusted appraiser. 

There are times I get a written quote from a licensed contractor ... and I pay them for their time.  

They are still in a state of change. I like this question. Let's see what is said

I would definitely contact any appraiser. If you do put something together for clients, remind them you're not an appraiser. 

It seems that all appraisers are a bit different in how they value things.  I have looked at numerous previous appraisals to get a range of how they value items.

In your unique real estate market that behaves differently than any of ours do, only the local profession can answer that for your zip code. Not like valuation of a car where does it have the V8 engine or premium leather and sound add or subtract. A fireplace, an extra garage bay, bath or bigger lot.. if your "old one" is out of date, stale, head into the MLS recent sales and study the market for the answers. You could try to lasso a local insurance agent or a tradesmen to pick their brains but market data is where you see the needed adjustments Danny Skelly to know in this neighborhood, in today's market with these plus and minus features and conditions what to tell the buyer and seller to expect in your Colorado pin point location.

RPR just conducted a survey on this a few weeks ago.

Check with your local Real Estate Association and see if they have a class coming up on appraisals or even the subject you are mentioning here.

I've seen things like this from Home Builders Assoc. but not in some time. I know I've posted about it but it is ancient now.

Candice gave you another source that looks like it might help also.

In Denver, I have taken classes (the appraiser was the teacher) and they go over the value given for items, and how to adjust.    In-person classes were really good, as lots of items were brought up.   With taking locally, you get the local adjustments.

ask a local appraiser

Personally, I'd farm this out to an appraiser.  It'll cost a few hundred, but you get someone who is doing this regularly and, most importantly, you get distance so if the client doesn't like the number it's not your number but the one the appraiser came up with.  Plus it's on their liability insurance now.

There are manuals ( can’t remember their names) that keep Appraisers updated on these issues 

It will take a bit of research in your local area. 

Here's what I use - you can search for your area as all areas have very different costs.