Sometimes Orvis will sell tartan trousers (not technically trews) for around $100 USD. I've also seen trousers in Government Sett (Black Watch) from Polo. Brooks Brothers might be another possibility.

I love trews myself; someday I'll get a pair in the Argyll District/Campbell of Cawdor tartan. They are a great alternative to the kilt for formal functions.

Regards,

Todd