My lender said the rate is only half the cost and I cannot stop turning that over

The waterfall comment your lender made is the whole game. On gap money specifically, the interest rate is almost a distraction. What matters is whether they have a participation kicker, and if so, whether it triggers on gross sale price or net proceeds after your first lien gets paid, because those two structures can be $8,000 apart on a Spokane-priced flip.

Your 18 to 19 cent effective cost math sounds right if there is a 20 to 25% profit split sitting on top of the coupon, which is pretty standard for unsecured or thin-collateral gap in a tertiary market like Spokane where the gap lender has less exit confidence than they would in Seattle or Portland. The number I would be pressing them on is whether the participation is capped, because an uncapped split on a deal that outperforms is actually the more dangerous clause than the one that hurts you on a short exit.

The profit split trigger is the clause nobody reads until they're in default on timeline.

Your math is not wrong, it is just missing the extension fee line, which is usually where the real bleeding starts. The lenders I have dealt with on the service side are not necessarily trying to take your upside on day one, they are waiting to see if you slip past month six, and then the extension is one or two points on the full note amount, not just the gap piece. On a $60,000 gap that does not sound catastrophic, but if the extension fee is calculated on the senior debt too because of a cross-collateral clause, that changes the number fast. I had a vendor I work with closely go through exactly this on a Spokane-adjacent flip in 2022, exit landed $22,000 light and the extension fees alone ate another $4,100 before the profit split even activated.

The one thing I have never seen work is negotiating the profit split percentage down after the fact once you are already past your exit window. People think they can get back to the table when the market softens and the lender will be reasonable about it. What actually happens is the lender already modeled for that scenario, it is in the docs, and they have zero incentive to move because time is now working in their direction not yours. The clause to fix is before you sign, not after you are underwater on a timeline.

The thing I keep coming back to with gap money is what the documents say about extension, not what the lender says verbally. Did you actually read the extension clause, and does it give them discretion to call the note or does it auto-extend at a penalty rate? Those are completely different instruments even if the headline numbers look identical. I had a situation where two offers were 12.5% apart on paper and about $40,000 apart in reality once you got to page nine of the term sheet. What triggers the profit participation, is it gross sale price or net after your closing costs and commissions?

Modeled the same $60k stack last spring and missed the default extension fee entirely, cost me $2,100 on paper before I ever closed.

Spokane exit timelines are running longer than people expect right now, inventory has loosened enough that you are not getting the same 30-day flip window you would have gotten in 2021 or 2022. That changes the gap math fast because most of those profit split triggers are not based on your return, they are based on calendar, and month seven looks very different from month five on a $60k piece. Get the lender to define exactly what "sale proceeds" means in their contract, because some count gross and not net after your closing costs, and on a $30k light exit that distinction alone can swing you another three or four points.