Basta JOIN con questa tabella USD :
SELECT sub.*, USD.USD
FROM
(
SELECT
t1.id_auction,
t2.bidder_1 AS bidder,
t2.item,
1 AS bidder_number
FROM table1 t1
INNER JOIN table1 t2 ON t1.id_auction = t2.id_auction
AND t2.winner_1 = 1
UNION ALL
SELECT
t1.id_auction,
t2.bidder_2 AS bidder,
t2.item,
2
FROM table1 t1
INNER JOIN table1 t2 ON t1.id_auction = t2.id_auction
AND t2.winner_2 = 1
) AS sub
INNER JOIN USD ON sub.id_auction = USD.id_auction
ORDER BY id_auction, bidder;
Dimostrazione SQL Fiddle aggiornata
| ID_AUCTION | BIDDER | ITEM | BIDDER_NUMBER | USD |
-----------------------------------------------------
| 1 | Alice | cup | 1 | 100 |
| 1 | Ben | cup | 2 | 100 |
| 2 | Alice | mug | 2 | 150 |
| 3 | Ben | pen | 1 | 50 |
| 3 | Charles | pen | 2 | 50 |
