I looked into spray foam.

@Dennis M. above used cellulose foam, which works great and is purpose-made for that application.

Most recently I paid $2.75 per square foot for 12mm thick with attached foam underlayment pad at a damaged freight store.

Also, the Zinus Memory Foam 12 Inch Mattresses are really nice.

Your prices are also conservative and you should be able to shave money off on a few of them.Appliances, cabinets, paint and even flooring you should be able to trim the costs down some (especially if you are doing material costs only).I can also say that if someone is getting all that work done, plus some electrical and plumbing for $6-7,000 that is completely unrealistic.

@Thomas S., have to consider local code for driveways which may not allow for gravel.

They were like $20 from Costco but it's the small things people notice.Also, costco mattresses that come in a box... they're inexpensive and VERY comfy, and they're memory foam which you can advertise in the listing. 

The Mastercard has a balance, and my Dollar Shave Club razors are charged to my Chase ($10 every other month). 

BUT if you do, and IF IT ACTUALLY does work out, the potential pay off is HUGE and can shave YEARS of Initial start up time off the investment clock.

Purchase price: $20,000 Cash invested: $329,000 Sale price: $473,200 Land Purchase in 2012, New Construction SFR built in 2016: Custom designed bespoke single family home. 3 bed 2.5 ba w/ roof deck, Curb-side Electric Car charging station, Sauna, 4 zone Energy efficient smart air system, HRV whole house fresh air system, Above code spray foam and batt insulation, Commercial grade White TPO roofing system, Triple Pane windows and rear sliding door, One of kind custom wood, metal and glass stairwell fabricated on-site, Phillips Hue smart light pockets throughout (compatible with Alexa, google home, Siri etc) 4 smart thermostats controllable remotely via app and Alexa, Google, Siri.