Here are queries that will give you buffer breakdown and plan cache breakdown per database. Note that plan cache is broken down by MB, not by pages - I don't know of a handy way, off the top of my head, to calculate the pages used by the plan cache (you'd think it's simple division, but not necessarily).
-- buffer pool breakdown
db = DB_NAME(database_id),
c = COUNT(DISTINCT page_id),
size_MB = CONVERT(DECIMAL(12,2), COUNT(DISTINCT page_id)*8.0/1024)
WHERE database_id < 32767
GROUP BY database_id
ORDER BY c DESC;
-- plan cache breakdown
db = DB_NAME(t.[dbid]),
size_MB = CONVERT(DECIMAL(12,2), SUM(p.size_in_bytes)/1024.0/1024)
FROM sys.dm_exec_cached_plans AS p
CROSS APPLY sys.dm_exec_sql_text(plan_handle) AS t
WHERE t.[dbid] < 32767
GROUP BY t.[dbid]
ORDER BY size_MB DESC;
On my test instance there are cases where plan cache is larger than the buffer pool for a given database.
The buffer pool contains both the data cache and the plan cache. But AFAIK sys.dm_os_buffer_descriptors only contains information about the data cache (index and data pages), so it does not necessarily reflect the entire buffer pool. The cases I saw above seem to support that theory.