You can do it for less using Avios to book American Airlines or US Airways flgihts if you can find it in 1-stop. SEA/PDX-PHX/DFW-CUN is 17,500 avios each way. SEA/PDX-ORD-CUN would be 20,000 each way.
You can transfer UR points to Avios.
However, I see nothing available even remotely close to your dates. This late in the year, you probably won't find much better than 50k on Alaska (although I couldn't find this either...shows up as 70k for me)