Trials 2 Second Edition is pretty fun IMO, I bought it in a Indie pack.
It usually costs $9.99, but now is 33% Off:
$6.69. There's a gameplay video on the website.
http://store.steampowered.com/app/16600/
There's a demo and some videos you can download to see if you like it.
Also,
Audiosurf, I have this game too and it's fun for people who like music, but there's not really a objective or something, just to spend your time playing "literally" on the track you're listening to
There's also a demo for it.
$6.69
http://store.steampowered.com/app/12900/
iFluid is very fun too, it uses PhysX and I like to play it, only problem is, it costs
$8.99 and I see you only have $5 to spend on a game. There's a gameplay video on the website.
http://store.steampowered.com/app/23200/
I have got
Xpand Rally and for me I don't think it's worth it, depends on the person who is playing it. Neither cars or tracks are real, it's got some very good graphics and some attention to detail but car physics aren't that good anyway. Even though graphics are very good maxed out, playing this game on low graphics will result in overall game looking like toy cars and "play dough" tracks. Well, look at the video and see if you like it.
http://www.youtube.com/watch?v=-Iqa0NNgaGg
IMO
GTI Racing is definitely not worth it.
Gravitron 2 is very fun IMO, I have it too. You can try the free demo before getting it:
$4.49
http://store.steampowered.com/app/21300/
Even though it is completely out of plans, a "cheap" game I would recommend is
World of Goo. It's very addictive and fun to play, I spent more than 2 hours just playing the demo
Was $19.99, this weekend only:
$13.39. There's a free demo for download.
http://store.steampowered.com/app/22000/