Surely there are guides on that. Read up on the basics and view how others are selling their websites and get a feel of what sells. Also if your business mainly focuses on website flipping, then when doing the numbers you might not need to flip a website daily to meet your expectations.
Your product is websites. So create some projections on the next year or 2 that you want your growth to be.
say you sell "X" amount of websites a day. And the cost of those websites is "Y"
You can then create some projections
ex. Day 1: Company buys 3 Websites were bought at avg $1.3k each total -- $3.9k initial investment.
Day 3 Sold those 3 websites (somehow) at avg of 1.45k each. Total
Revenue = $4350
Expenses = $3900
Net=$450
Of course if you have legal services, accountants, employees, insurance you would also factor in that in expenses in addition.
It's difficult to suggest where to start when there are so many factors budget constraints even though its just website flipping.
Basically in my opinion its up to the Chief of what you want, instead of flipping 3 websites at 1.5k each maybe you just want to flip 1 site at4k and save the head ache of selling 3 websites, and only have 1 website to sell.