Tuning SQL to drop execution cost

SELECT n.MSISDN,
(SELECT ptc2.PRIMARY_ACCOUNT_NUMBER
FROM p_topup_cards ptc2
WHERE ptc2.NUMR_MSISDN = n.MSISDN
--AND ptc2.CARD_TYPE = 1
AND ptc2.PRIMARY_ACCOUNT_NUMBER LIKE '894428%'
AND ROWNUM < 2) pan
FROM numbers n  ,p_number_history pnh
WHERE n.MSISDN = pnh.NUMR_MSISDN
AND n.STATUS = 'A'
AND n.Barred = 'N'
AND n.spid_spid = '416'
--AND n.first_transaction IS NOT NULL
--AND pnh.END_TIMESTAMP IS NULL
AND pnh.PLFM_PLTP_CODE = 'IN'
AND ROWNUM <= 2000

Plan

SELECT STATEMENT  FIRST_ROWS Cost: 758,319 Bytes: 72,000  Cardinality: 2,000

3 COUNT STOPKEY

2 TABLE ACCESS BY INDEX ROWID LIVEUSER.PREPAY_TOPUP_CARDS Cost: 6  Bytes: 32  Cardinality: 1

1 INDEX RANGE SCAN UNIQUE LIVEUSER.PPTC_PK Cost: 4  Cardinality: 1

8 COUNT STOPKEY

7 NESTED LOOPS  Cost: 758,319 Bytes: 8,591,616  Cardinality: 238,656

5 TABLE ACCESS BY INDEX ROWID LIVEUSER.NUMBERS Cost: 46,110  Bytes: 4,748,060  Cardinality: 237,403

4 INDEX RANGE SCAN NON-UNIQUE LIVEUSER.NUMR_SPID_FK_I Cost: 3,682  Cardinality: 949,610

6 INDEX RANGE SCAN UNIQUE LIVEUSER.PFM_NUM_HS_PK Cost: 3  Bytes: 16  Cardinality: 1

Initial Analysis
This SQL needs to be re-written to avoid any join and mainly try and avoid statement “n.spid_spid = ’416′”

Why?
This simple stmt “select * from numbers where spid_spid = ’416′;” and it shows the cost of 46K+

Plan

SELECT STATEMENT FIRST_ROWS Cost: 46,110 Bytes: 40,833,230 Cardinality: 949,610

2 TABLE ACCESS BY INDEX ROWID CPI_SYSTEM.NUMBERS Cost: 46,110 Bytes: 40,833,230 Cardinality: 949,610

1 INDEX RANGE SCAN NON-UNIQUE CPI_SYSTEM.NUMR_SPID_FK_I Cost: 3,682 Cardinality: 949,610