As for flights, the UAExpress looks okay at the moment with about three rows wide open in E+. The LHR flight is pretty full but is showing the A/B pairs in rows 22 through 26 still open. (Config is 2x3x2) Return the E+ cabins are both wide open, including on the CDG flight the centre seat sections. (When traveling as a couple, UA will not let you book the C and E seats keeping the D one open.) But there are five rows of A/B and six of F/G pairs open.

Hope this helps with your decision.

foxhunter, the actual amount depends on the distance flown. You can opt for paying by segment (you'd have to contact UA to determine those) or purchase a one-year "membership" in E+ which is what the OP would be doing. Then all your future flights on UA will have access to those seats for a 12-month period of time at no additional charge. (Or you can earn Premier status with 25K elite qualifying miles, or a set number of segments.)

pilotboy, feel free to PM me if you need more details. I've archived the seating charts for you and can email them as an attachment you should be able to see.