Du kan använda SQL CASE för det:
SELECT price, currency,
CASE WHEN currency='USD' THEN price * 1
WHEN currency='RUB' THEN price * 65
ELSE price
END as final_price
FROM products
WHERE final_price BETWEEN 10 AND 100
Rails version:
Product.select("price, currency, CASE
WHEN currency='USD' THEN price * 1
WHEN currency='RUB' THEN price * 65
ELSE price END as final_price")
.where("final_price BETWEEN ? AND ?", 10, 100)