i'm surprised i haven't seen anybody mentioning MEX duty free. $8USD bottles of bacardi products, lots of really great high-end tequila, too. my favorite was bottles of centenario reposado for $15USD ($50+ in the states).
ORD duty free has smirnoff black label (small kettle-made batches in the UK) for around $20USD, another great product i wish i could get more.