What a sandbox serves is billed as outbound traffic, and persistence costs nothing extra

For sandboxes created from today, what a sandbox sends back through a preview, a custom domain or a TCP port counts toward the account's 100 GiB a month free, then $0.02 per GB, like what it sends over its own connections; what visitors send in stays free. While a sandbox made from today runs, its disk past the first 10 GiB is billed at $0.08 per decimal GB per 30-day month, as a paused disk is; a disk of 10 GiB or less never pays it. A persistent sandbox no longer pays $0.11 per GiB for a reserved disk all the time: its disk is billed as any sandbox's, and persistent means only "keep it running". See pricing.