The floor you heard about is roughly right in shape. AppFolio's residential product is priced per unit per month with a minimum monthly charge in the high two hundreds to around three hundred dollars, so a 40 door operator pays the minimum and effectively subsidizes the platform until they're somewhere around 200 units. Buildium starts far lower, in the fifty to eighty dollar a month range on its entry tier for small portfolios, and rises with unit count and features. DoorLoop, TenantCloud and Rentvine occupy similar territory with different feature trade-offs. Those numbers move, so pull current pricing from each vendor directly before you build a model on them.
Software isn't the whole stack. Budget separately for tenant screening, usually $30 to $50 per applicant and almost always charged to the applicant rather than absorbed, electronic signature if it isn't bundled, a business phone system, and bank fees on the trust account. Some platforms charge extra per e-signed lease or per ACH transaction, and the transaction fees are where a small operator gets surprised.
The number that matters more than the monthly price is what happens when you switch. Migrating owner ledgers, open work orders and security deposit balances from one platform to another is a real project, often several thousand dollars if you pay the vendor to do it and several weeks of your own time if you don't. Small managers often pick the cheap tool, grow into needing the expensive one, and then discover the switch costs more than two years of the price difference. Ask each vendor what export formats they give you on the way out, in writing, before you sign anything.