I'll throw my 2 cents in in the form of the Big Dirty Live Game List:
Amped 2: (I paid $6.48, Target Clearance) Never Opened it, can be found on the cheap anymore.
Bicycle Casino: (I paid $9.99, can be found used sub-$20 most places): The live community is active with games of Texas Hold Em. You can earn money in single player games to bring online, which helps out many situations.
Colin McRae 2005: ($9.99 EB, still eligible for Price Grabber Rebate): Good rally sim, next to no Live! community. Only ghost racing available.
Counterstrike: ($9.99 EB, was recently on clearance at Target, PH game): Just like the PC Game, active community.
DDR Ultramix/Ultramix 2: UM-1 is a PH, but live community is next to non-existant, not a cheap game series to get into, since you really need a pad.
Dead Or Alive Ultimate- Can be snagged for $30, less if you poke around. Community is full of the hardcore DOA fanboys, and don't take well to "noobs".
FIFA 05- It's approaching the end of it's shelf life. Can be had for $20ish used, $30 new still. Community has started to die off some, but games can still be found.
Forza- Recent price drop, active community, best racer on the console period.
Ghost Recon- PH, recently on Target clearance, semi-active community
Guilty Gear X2# Reload- Cheap, good 2D fighter, but Live is a barren wasteland.
GunGriffon Allied Strike- I nabbed for $10 off EB over the summer, combo Mech combat and FPS, never saw anyone on Live
Links 2004- Can be found really cheap, great golf game, slightly active community, can find games at most times
MLB 2K5- Can be had for $15, good baseball game, vibrant live community, probably will dwindle past the world series
MLB Inside Pitch 2003- If your Circuit City still has it, its 99 cents, and barely worth that. Bad game, no Live players
Mortal Kombat Deception- Not quite cheap enough if you didn't get in on the 9.99 EB sale of it. Lots of MK games going on, but Puzzle Kombat is pretty dry
NHL 2K6- New Game, Under $20 (GGC it for $15), new NHL rules implemented, numerous players
NHL Rivals 2004- Barren wasteland of a game that can be had for peanuts
Outlaw Volleyball- This is another game that can be found for scratch, and there is no life to it online at all.
Outrun 2- Can find this around $10 used, and the community has been on a steep decline over the past few months.
Project Gotham 2: PH that can be found cheap used, not as vibrant of a racing community as Forza, but there's still some life.
Rainbow Six 3: This can be found near scratch, used or new. $10 is really the max that should be laid out. Live community exists, but its full of the hardcore to the franchise.
Rallisport Challenge 2: Sub $20 new, can be found fairly cheap used. An arcade rally game that has only one or two races going on Live at once. A dwindling community.
Sega GT Online: I've seen this for $5.99 used before. It's SegaGT with an online component that I haven't seen used since the game was new in Feb 2004. Calling it a barren wasteland on live would be being nice.
Star Wars: Clone Wars/Tetris Worlds- This Holiday '03 disc can be found used really cheap, but the two games on there have little to no action. I've seen someone occasionally play Tetris Worlds, but never came across anyone playing Clone Wars.
Street Racing Syndicate- Was part of the Circuit City $9.99 clearance. Haven't seen too much movement on price, used hovers around $15. There are a handful of live players, but a racing dollar is better spent elsewhere.
XIII- This can be found for scratch, but the Live community I've found to be a crapshoot. Sometimes several people are playing, others just dust and tumbleweed.
Top Spin- Can be found pretty cheap, especially on the Holiday '04 bundle disc. Community tends to be like that of Links, can usually find a game, but sometimes it dosen't happen.
Unreal Championship- PH game, can be found for scratch used. Live games I've found to be few and far between.
Unreal II: The Awakening- Team based multiplayer component, occasional live games. It comes to being lucky finding it for less than $10 though.