There is a floor, and it comes from data volume.
Clicks on seller-intent searches are expensive because investors, iBuyers, and agents all bid on the same handful of phrases. In many metros a click runs somewhere between $15 and $60. It commonly takes 30 to 80 clicks before one person calls or fills in a form. At $30 a click and 50 clicks, one lead cost $1,500. So $800 a month buys you part of a lead, and on top of that your daily budget will be spent by mid-morning, which means you're invisible for the hours when people actually search.
The other half is that the platform's bidding uses your own conversion history to decide who to show ads to. With two conversions a month it never gets past guessing, so your cost per lead stays at the bad end of the range instead of settling down.
Then there's the fee ratio in your quote. $1,500 of management on $800 of spend is $2,300 out for maybe one lead. That's the number to push on, not the minimum itself. Some agencies price at a percentage of spend with a floor, which behaves better on small accounts.
If $800 is what you have, ask them what their smallest currently running account spends and how many leads it produces per month, in writing. And know that $800 of direct mail or cold calling buys a lot more attempts at small multifamily owners than $800 of paid search does. The labor is yours instead of theirs.