The break even framing is sound as far as it goes. Two things are doing more work in it than they look.
The first is the 30 days. You've treated the time saving as a known input, and it's an estimate. The general pattern that well presented properties sell faster and for more than comparable unstaged listings is well documented, but the size of that effect on one specific house in one specific market isn't something anyone can hand you. The listings that get staged also tend to be the ones with motivated sellers, good agents, and correct pricing, so published comparisons flatter staging somewhat. Treat your 30 days as a scenario and run 0 and 15 alongside it.
The second is the term. As @parcel says, $3,500 buys a defined window. Price the full engagement at your realistic marketing time, not the minimum, and include the de-stage labor if the quote separates it.
One more thing sitting outside the spreadsheet. Staging changes the photos, and the photos change showing volume, which is where a lot of the effect actually shows up. That matters on a flip because your first two weeks of traffic are the strongest you'll get, so a stager who can't install before the photographer arrives has cost you most of the benefit whatever the fee says. Lock the install date to the photo date in writing, ahead of listing. If your inventory turns are seasonal in your market, a stager may also be booked out weeks in advance, and waiting for a slot is carry you're paying at $2,100 a month.
