To look at the average daily bytes used in the last days, you can use stage_storage_usage_history:
select *
from table(information_schema.stage_storage_usage_history(dateadd('days',-10,current_date()), current_date()));
If you want to look at the detail from stages and databases:
select convert_timezone('UTC', usage_date) as usage_date
, database_name as object_name
, 'database' as object_type
, max(AVERAGE_DATABASE_BYTES) as database_bytes
, max(AVERAGE_FAILSAFE_BYTES) as failsafe_bytes
, 0 as stage_bytes
from snowflake.account_usage.database_storage_usage_history
where usage_date >= date_trunc('day', ('2021-12-01')::timestamp_ntz)
and usage_date < date_trunc('day', ('2021-12-05')::timestamp_ntz)
group by 1, 2, 3
union all select convert_timezone('UTC', usage_date) as usage_date
, 'Stages' as object_name
, 'stage' as object_type
, 0 as database_bytes
, 0 as failsafe_bytes
, max(AVERAGE_STAGE_BYTES) as stage_bytes
from snowflake.account_usage.stage_storage_usage_history
where usage_date >= date_trunc('day', ('2021-12-01')::timestamp_ntz)
and usage_date < date_trunc('day', ('2021-12-05')::timestamp_ntz)
group by 1, 2, 3;
As Greg says in a comment, you can transform these monthly average bytes into credits with a formula that will depend on the specifics of your account contract.