A buy-out is a pre-agreed contractual route to stop future monthly SUS charges for that hire.
The buy-out value, or the basis for calculating it, is agreed with you in writing in your commercial terms at the outset, for each hire. You can exercise that pre-agreed option later under those terms, without a fresh commercial negotiation. Once it is paid, future monthly billing for that hire stops. Other hires are unaffected. Monthly fees already paid are not deducted, credited or refunded against the buy-out by default; they cover the months already elapsed.
It matters most over a long tenure: it is a contractual route you already hold, so you can close out future monthly charges for that hire without opening a new negotiation. It is a route to close out future monthly charges for one hire, not a cap on what recruitment can cost.