How is burrow RAM billed? Does it include the page cache?

The dashboard shows my burrow using 1.8 GB, but my process uses about 300 MB. Is the Linux page cache counted toward billed memory?

Billing uses the memory size you chose, not what is in use — usage graphs are for you, not the invoice. The page cache does show in the graph, which is where the 1.8 GB comes from.