As loyalty programs would have it, I am completely sucked into UAs MP and often question myself why I am hanging on. Below are some points to consider.

1. $800 for a 15 hr flight...absolutely worth it. I have never had cash upgrade options for anything less than 1.5K one way on that route despite booking W+ fare.
2. If you have paid a Y/B/M fare or lets just say your return fare costed you over $3000 then I would pass. for net of $4600 its a fare at par and you are not really getting a "deal".
3. Consider this - for $800 more you will get closer to your PQM requirements which can get you some benefits that only you can justify. if it gets you gold or plus I would say go for it as E+ seats will help you recover. Plat gets your RPUs - worth much more if you put to good use.
4. Dont waste too much time ...you never know that $800 upgrade changes to $2500 and all this education may go down the drain.

Good luck and remember this is coming from a guy who has flown UA48/49 6 times last year in economy and when they throw bags of snack on your face you will realize how much that $800 could have got you in the front.