Without trying to give you an answer, know that you can always start even smaller with a one word field and answer. Here's something I tried:
1) Create a test custom form, and a test dropdown field with options for 1 and 2.
2) Click on Apply
3) create a calculated custom field on the test custom form and populate it with this:
IF(test=2,CONCAT("two"),"")
Add to an object and change the toggle to "2". Once you save, the calculated field populates with "two". So now you know the syntax. Once you know the syntax, start making your changes and work one change in at a time until you fail -- this way you know exactly what is failing.
For example, my next change was to populate one of the dropdown options with Project Value 1000 - 9999. This produced an error when I went to plug it into my calculated field, so I couldn't even save it. I put it in double quotes and that seemed to work ("Project Value 1000 - 9999") -- toggling to that value caused the calculated field to repopulate with "two". So now you know that a workable syntax with this value is double quotes. (single quotes would also work, the important thing is, you now know it works with a really low impact formula, so keep building)
So now, tackle the calculation (the prod part).
For the PROD part, I started with IF(test="Project Value 1000 - 9999",PROD(5,6),"") and got that working (my test form changed to 30), so now I know I have the right syntax there. Then I can trade in for a fieldname -- let's say a number field called "test2". If that one works, then now you can try the Total Installation Cost field.
Anyway, long story short, what I eventually found out on my instance is that your calculation seems fine. None of your fieldnames need "anything extra". They're fine.
You're basically down to IF(YourFieldname="some sort of answer",PROD(YourOtherField,.14),"I would always add something here to test with as well")
In other words, IF (Something, Do something, Otherwise do something else).