I've seen the actual cost to be around 15%-20%.

Given that the property was worth $390K in its current condition, I saw an opportunity to make the numbers work while keeping my upfront costs low.Instead of increasing my cash offer, I negotiated a $25K down payment with the seller carrying the balance for two years through owner financing.

In one of my markets assessments are performed every 3 years, and the cost of 3rd party appeals will cost 30% of 1 year savings. 

EDUCATE YOURSELF - yes, it will take time, but will lead to a selection that better meets your expectations & avoids potentially costly surprises!

The family viewed the house, loved it, and requested that I install satellite TV and add it to the costs, which I did.

Two of the kitchen cabinets were also cut in half.

I highly recommend you study this, practice it, and apply it.https://www.biggerpockets.com/blog/rental-property-cash-flow...Property Details:Type: Single-family homePurchase Price: $289,900Down Payment: 20% ($57,980)Loan Amount: $231,920Interest Rate: 6.85%Loan Term: 30 yearsMonthly Mortgage Payment:Using a mortgage calculator, your principal and interest will cost $1,515 per monthRental Income:Average Rent for Single-Family Home: $2,171 per monthOperating Expenses:Property Taxes: $300 per monthInsurance: $100 per monthMaintenance and Repairs: Budgeting 5% of rental income ($109 per month)Vacancy Rate: Assuming 5% vacancy ($109 per month)Property Management Fees: If outsourced, typically 10% of rental income ($217 per month)Total Monthly Expenses:Operating Expenses: $835Mortgage Payment: $1,515Total Expenses: $2,350Net Monthly Cash Flow:Rental Income: $2,171Total Expenses: $2,350Net Cash Flow: -$179 per monthConclusion:In this scenario, the rental income of $2,171 per month does not fully cover the total monthly expenses of $2,350, resulting in a negative cash flow of $179.

Figuring out the true cost will heavily depend on the type of dig down, depth, and total square footage. 

There are a few more “big ticket” items that need to be considered (Minisplits & landscaping) but most of the major costs have already been accounted for on this property.We currently have a 30 year conventional mortgage with a 7.125% rate on a purchase price of $750k. 

I might be biased - though the easiest way to tell which option works best is to get a side-by-side comparison so you can see the overall costs, payments, etc.