Rough ranges, and you should confirm current pricing directly with each vendor since they change tiers regularly.
Dynamic pricing, meaning software that adjusts your nightly rates daily based on demand and comparable listings, is usually around $20 to $25 per listing per month, with the per-listing price dropping as you add doors. PriceLabs and Wheelhouse are the two names you'll hear most.
A property management system, sometimes called a PMS or a channel manager, is the tool that syncs calendars across Airbnb and Vrbo so you don't double book, keeps one inbox, and triggers automated messages. Pricing varies a lot. Hospitable sits at the lower end, often $30 to $40 per listing. Hostaway and Guesty are aimed at bigger portfolios and typically start around $40 to $100 per listing with a monthly minimum and sometimes an onboarding fee. OwnerRez charges per property with a low base.
At three listings you can genuinely skip the PMS if all three live on one platform, because Airbnb's own scheduled messages and calendar handle a small portfolio adequately. What I'd not skip is the pricing tool, since the revenue difference from getting rates right on shoulder season is usually larger than the subscription many times over.
Hardware is the cost people forget. Smart locks run $150 to $300 per door, and noise monitoring sensors are roughly $100 to $150 per unit plus a small subscription. Decide with your friend in writing whether the owner buys those and owns them, or you buy them and take them when the contract ends, because that question gets ugly at the exit rather than at the start.