I would recommend using www.easysabre.com and play around with fares between UA Express communities on one side of the country connecting to a hub to a UA flight which requires connecting in ORD or DEN to another UA flight which them requires one more UA Express to your final destination.
An example: Wenatchee, WA to Seattle to Chicago to Boston to Portland, ME.
Regardless, I doubt you will find a 14 or 21 day advance purchase fare more than $600.