RedPill said:
Detroit and Minneapolis are both Northwest hubs, therefore cheap tickets between them.
exactly.
for cheap domestic tickets I always use Farecast.com to find 'em, then book with the airline itself online. Also, if I know that one endpoint is a Southwest hub, I check them too, since they don't usually show up in aggregators (for example, I fly into BWI fairly often).
unfortunately I don't know jack about international flights -- the price has always kept me out of that market.