What's First Class on an International Flight Worth?

This is kind of amusing. I'm flying from SFO to Frankfurt today, and after much searching and fretting, I ended up buying a business class seat with my "Mileage Plus" miles that I've been accruing over the years. A coach seat would have cost $550-ish, and those fares are non-upgradeable -- i.e., there's no amount of miles or money that can sneak you into business class, because of the fare structure. So I was randomly looking around United's website to see about buying the ticket with miles, and I saw that a business class seat cost 80,000 miles, which I had. The seat would have cost like $1500 in money, so I figure that's a pretty good bargain, and besides, that airline's probably going under any minute now so I should use these miles while the gettin's good, right?

So I went to check in online today for the SFO-Frankfurt flight, and after giving my info, the website asked me "Would you like to upgrade to United First?" Whaaa, hummmm? The flight is on a big old international 777, where business class is pretty good but not the super spread-out business class like on some planes I've been on. So I click the "continue" arrow to check it out. The next screen informs me that "Upgrade to First Class for: $619." Gulp! So, wait, let's figure this out. A round-trip first class ticket probably would have cost like $5,000, and if you assume 100 mileage miles=$1, my flight "cost" $800. Multiply $619 x 2 (if it gave me the round trip option, which who knows if that would have happened), and you have a grand total of about $2000 for a first class ticket. Of course for God's sake there's no way I'm spending $600 for a one-way first class upgrade, but for a brief moment, the possibility was thrilling.

So I guess the question is: how much would I have paid? A 12-hour flight from SFO-Frankfurt, I'm already in business class although I got it for "free" with miles, a one-way upgrade to First... hmmm. Maybe $150, $200? I mean, those are lie-flat beds I think, that's worth about the cost of a night in a hotel, right? Well, anyway, of course I'm mostly just interested in getting there in one piece.