It is essentially calculated as the interest rate times the outstanding principal amount of the debt. Interest expense on the income statement represents interest accrued during the period covered by the financial statements, and not the amount of interest paid over that period.
Even though my basement bathroom cost was around $3,700, you could probably do it for just under $2,000 if you really pushed the envelope, maybe even less. I'm not going to sugarcoat the project. From a time perspective it takes awhile if you're doing it by yourself.
1 If not included, premade vanity countertops start at $20-$150 but can cost $200-$1,000 or more, and bathroom sinks start at $20-$70 but can cost $450-$750 or more; for details see How Much Does a Bathroom Countertop Cost and bathroom sink. Semi-custom or custom vanities can cost $200-$1,000 or more to install, depending on the complexity of the project, raising total costs to $700-$3,800 or more.