The short answer
At 300 units most US property management software costs $300 to $600 per month before one time fees. Yardi Breeze, Rent Manager Basic and Propertyware Basic land at about $300, AppFolio Core is reported near $447, DoorLoop Pro lists $394 billed yearly at 299 units and quotes 300 and up, and Buildium's plan floor plus one online rent payment per unit puts it near $580 to $600.
Letting costs $224 per month at 300 units billed yearly, the lowest subscription in that list. Month to month it is $449, which is more than the $1 per unit plans. Letting does not collect rent, screen tenants or list vacancies: tenants keep paying into your bank account and Letting reads the bank export.