One easy join.
Prerequisite: Dataload DS ccy -> CP ccy.

Code

def qapi = api.queryApi()

def t1 = qapi.tables().productExtensionRows("Costs")
def t2 = qapi.tables().companyParameterRows("Currencies")
def today = new Date()
def currency = "USD"

return qapi.source(t1, [t1.sku(), t1.Cost, t1.Currency])
        .leftOuterJoin(t2,
        { cols -> [t2.ExchangeRate, cols.Cost.multiply(t2.ExchangeRate).as("CostUSD")] },
        { cols -> qapi.exprs().and(t2.CcyFrom.equal(cols.Currency),
        t2.CcyTo.equal(currency),
        t2.CcyValidFrom.lessOrEqual(today),
        t2.CcyValidTo.greaterOrEqual(today),) })
        .take(10)
        .stream { it.collect { it } }

Result

sku	Cost	Currency	ExchangeRate	CostUSD

sku	Cost	Currency	ExchangeRate	CostUSD
10000001	69	GBP	1.26	86.94
10000001	64	CHF	1.12	71.68
10000001	73	GBP	1.26	91.98
10000001	87	CHF	1.12	97.44
10000001	37	DKK	0.14	5.18
10000001	3	USD
10000002	4	USD
10000002	53	USD
10000002	28	EUR	1.05	29.4
10000002	85	USD

Currency Conversion

Code

Result

See Also