Update with a Select Subquery

Hi folks.  I am attempting to update one table with sum aggregates of two other tables joined, where Meta_ID from the two stat tables matches the customer ID from the table I am updating.  Here is my script:

UPDATE gcgmkt.dbo.usr_fulldb_balances
SET gmg_PtsBal = (SELECT cds_statbalance.Meta_ID, ((sum(cds_statsummary.PtsEarned) + sum (cds_statsummary.PromoPtsEarned)) + (sum(cds_statbalance.PtsPromoAdjusted) + sum(cds_statbalance.PtsMgrAdjusted) + sum(cds_statbalance.PtsBalanceForward))) - (sum(cds_statbalance.PtsRedeemed)) as PtsBal
FROM gmgoasissql.WinOasis.dbo.CDS_STATBALANCE as cds_statbalance
    FULL OUTER JOIN gmgoasissql.WinOasis.dbo.CDS_STATSUMMARY as cds_statsummary
    ON cds_statbalance.IDType  = 'P'
      and cds_statsummary.IDType = 'P'
      and cds_statsummary.StatType = 'SLOT'
            and cds_statbalance.Meta_ID = cds_statsummary.Meta_ID
Group By cds_statbalance.Meta_ID)
WHERE gcgmkt.dbo.usr_fulldb_balances.gmg_player_id = Meta_ID

When I run it, I get:

Server: Msg 207, Level 16, State 3, Line 1
Invalid column name 'Meta_ID'.

If I add cds_statsummary in front of the final Meta_ID, I get:

Server: Msg 107, Level 16, State 2, Line 1
The column prefix 'cds_statbalance' does not match with a table name or alias name used in the query.

I get the feeling looking at other posts that I need some sort of additional join operation between the target table and the subquery result set, but it's a bit beyond me.  Help a fella out!  Thanks.