Listings for SMM panel scripts quote a single number: the licence. That part really is one-off, but it is the smaller half of the bill. A script is an archive of code, and on its own it does nothing until there is infrastructure underneath it.
After buying, you rent a server and set up a web server, PHP and a database on it. You buy a domain, issue an SSL certificate and keep renewing it, schedule background jobs, lock the admin area against password guessing, arrange backups and find somewhere to store them. None of this is one-off work: the server needs security patches, the engine needs version upgrades.
Providers are a separate line item. The script gives you an order form, but you build the service catalogue yourself - find providers, fund a balance with each one, wire up their APIs, watch stock levels and make sure their prices have not drifted away from yours. Payments work the same way: a gateway has to be connected and pass its own review, which means having legal pages and an identifiable seller.
And there is one cost no price list mentions - being on call. If the panel goes down at three in the morning, you are the one bringing it back up. Customers sitting on paid orders will not wait until you wake up.