You can find audiobooks online sometimes...but the pickin's are slim
I know what you're syaing though. Audiobooks are ridiculously expensive!!!
Idk if you're a Trek fan but the Next Generation series dvd boxsets come with an mp3 audiobook disc with about 4 hours of audiobook material. I'm a casual fan of the show but they seem good from the quick listen I had of one of them for the third season. Just throw it on your Ipod and you're good to go.