The short answer
Buildium's own pricing page lists Essential from $62, Growth from $192 and Premium from $400 per month, and the price climbs as your unit count rises. On top of that you pay per transaction and per feature: $2.35 per incoming EFT payment on Essential, $99 for each bank account you connect, and a paid inspections add on below the top tier. Letting charges one price per unit band, $59, $189 or $449 per month, and nothing per payment, because it does not process payments.
If you want tenants to pay rent inside the software, screen applicants and list vacancies from the same system, Buildium does that and Letting does not. Choose on that line.