I see leads with vacant houses on FSBO sites. 

High leverage enables rapid growth, no leverage is safe and enables peaceful sleep.

Apartments.com leads are usually pretty good as well but I don't get as many leads there as I do the other two websites.

Not possible.The only way to reliably get in touch with motivated sellers is for you to make it easy for them to find you.Get a solid website, work on the 3 pillars of conversions:1.

At the end of the day vet these lead gen sites/companies on a short-term basis, then flip to something else if the leads are not working while having the right expectations.

This is what I did for a cabin I built in Gatlinburg:  We took a few royalty-free photos of the area off of the internet, and submitted them to an art site online whereby they make a painting of the photo.  

Besides land, infrastructure/site work, and the building construction itself, am I missing any other major category of costs?

It's listed right on our website on the pricing page.

I watched my parents flip houses since I was little and have always been on job sites or made "pocket money" by doing chores like painting fences or washing kitchen cabinets, which I realize now my parents were putting us to work to build that sweat equity.

Web sites that have been making recommendations for years suddenly claim it is AI.