Okay, I'm thinking Bad Company 2, or Killzone 3.

For single-player: Resistance
Multiplayer: Battlefield 3 (fairly steep learning curve but when you get into it, it's incredible)

Seeing as you aren't getting Battlefield 3, then get either:
Killzone 3

Resistance FoM/2 (should both be very cheap by now)

Battlefield: Bad Company 2

Half-Life 2 if it's single player you are after. A fantastic game that every honest gamer should play. And you can find it in the Orange Box with the two expansions for dirty cheap.

There's not many shooters that are worth it for the single player only. You rush them through in 8-10 hours an that's it. Halfo-Life 2 and Bioshock are exceptions, they're masterpieces. Shooters are now about multiplayer. And the game to get for an amazing multiplayer is of course Battlefield 3 (I would say Enemy Territory: Quake Wars since it's the best online shooter ever made, but the console versions were poor and you can't find players online anymore). Killzone 2 would be my second choice, I love the ultra heavy controls in that one.

Stick with your initial thoughts and go Killzone 2 or 3... If he cares about the story id say start at Killzone 2 then three, although if he just wants a whole lot of fun Killzone 3 is the better game imo. Interesting story much easier shooting mechanics, amazing graphics and just an awesome campaign best iv'e played on a fps.

& for the online its also very fun with lots of stuff to unlock and plenty of time to waste in it, with vehicles and the whole shabang.

Killzone 3. Me and my brother went through the whole campaign together (2-player split-screen) and it was amazing. Multiplayer is also fun and addictive. Battlefield 3 is decent. I got it on sale for $29.99 during black Friday, so shop around to see if any stores will be offering it at that price again.

Not sure where OP is getting the information that BF3 is 60 bux. Here is the limited edition which comes with the BTK addon for free (15 bux otherwise) for 40 bux. Unless he's from another country.

http://www.newegg.com/Product/Product.aspx?Item=N82E16879200840

I'd say get Killzone 2 over Killzone 3, it has a better singleplayer. Graphics are the same. Controls are a bit heavier unlike other FPS, so know what you're getting.

The online community is pretty dead though. KZ3 has split-screen for singleplayer and a little less dead community, so you'll have to decide for your own.

Ok OP let me break down your options.

Multiplatform games with mid to large communities under $40
-Battlefield Bad Company 2(a lot of the community migrated to BF3)
-Call of Duty Black OPs(still a huge community even with release of MW3)
-Homefront

Exclusive games with mid to large communities for under $40
-MAG
-Resistance 3(small community)
-Killzone 3
-Killzone 2

well you did ask for the best! also i recomend Resistance 1-3, Bulletstorm, Army of 2: 40th day(TPS), and Vanquish(TPS).

i think you said something about Metal GEAR4. it's not an FPS, it's a TPS. Rage is good but not impressive, and Crysis2 is very intense. KZ3 can be unforgiving at times.

Killzone 3 & Resistance Series