I don't know JC, you might be the one trolling here. People have come up with a number of well-received, popular examples and you've dismissed them all as not fitting into the FPS genre or not counting for some other tenuous reason.
All I've got are tenuous reasons. By design...
Most examples are not modern FPS games. Let's clarify and say 2009 and later. That eliminates BioShock and Half-Life 2.
To even offer Portal or its sequel as an FPS is silly. Furthermore, you talked about paying $60 for Portal 2. It hadn't been out 2 weeks, and it was reduced to $39.99 for a month, IIRC. Actually, that's still the price on Amazon. How do you feel about that? Keep in mind that Portal 2 is a full-fledged game, not a remake of a 10-year old game. I think $40 for Portal 2 is an amazing value. I also think that it's probably what the price point should have been initially, making a "sale" of it unneeded.
Niterider came closest with Metro 2033.
So, I'm illustrating why "Perfect Dark isn't Halo" isn't a valid argument by using "________ isn't Halo" when applicable. In this case, Metro 2033, which has been the only decent example offered up as a modern FPS with no multiplayer component.
In closing, I'm pulling out my last card in this whole debate of just why multiplayer is being left out:
I give you Marathon: Durandal.
A port of Marathon 2 to the Xbox Live Arcade was announced at the 2007 E3 Conference by Microsoft. The Xbox Live Arcade version of the game was developed by Freeverse Software, and presents a revised HUD and provides support for both 4-player split screen play on the same console and 8-player Xbox Live gameplay. Since the old Marathon network code could not handle reliable internet play, the ReplicaNet middleware was used which also allowed coop play with up to eight players. The game also supports 16:9 screen resolutions, high-definition output at 60 frames per second (compared to the original 30) as well as upgraded models and graphics, support for 8-player co-op mode over Xbox Live, several international versions, and a new game mode called "Survival" where the player is given a large amount of weapons and ammo and must defeat endless waves of enemies, earning points for effectiveness and taking minimal damage, with high scores shared over Xbox Live.
The original game did not support WAN multiplayer:
Like Marathon, multiplayer can be played with eight players over a LAN connection.
So, little iOS app developer Freeverse can handle it, but the strain on resources at 343 was just too great. Not buying it.