Was playing around and noticed UA & US have matching sale fares SEA-MEX for fall travel. Since US allows many more connections options via Eastern USA, I worked up a fare that came in at approx USD 345.00 inc tax return. I say approx because I get a CAD rate with tax and just roughly calculated in the ROE. Here's what it looks like:

US6171 V 22Sep SEA 0605 SFO 0803
US6879 V 22Sep SFO 0841 DEN 1217
US7064 V 22Sep DEN 1353 ORD 1720
US6480 V 22Sep ORD 1830 BWI 2126
US877 V 23Sep BWI 0740 CLT 0900
US629 V 23Sep CLT 1015 MEX 1317

US5833 V 27Sep MEX 1510 ORD 1907
US6047 V 27Sep ORD 2135 SEA 2348

The fare basis is VXE32MWN, a webfare, or VXs107WH which is a nonwebfare but exactly the same price. Travel permitted Mon-Thurs both directions and the both fares have a 2 day min. stay, 30 day max. stay. Being a TA with CRS access, you may have to call the airline or work out alternate routings/ connections on the web to get this price. But not a bad special from the Northwest to Mexico even for a quick weekend and I thought it worth sharing.