i know that there's the regular bid where everyone spends insane amounts of karma for a day(?) of advertising and the one hour bid where it's a fixed cost of 100 kp and whoever's the fastest gets the advertising space for that hour. i'm not sure if you get your kp back if you don't win the bid though
you could also take advantage of advertisement shops who usually have packages of, say, 100 wall posts for some amount of karma. then again, you could always do self-advertising which doesn't cost anything unless the user has the 1 kp transfer feature activated

i haven't actually purchased any advertising from shops or used the bid calendar, this is just from what i've seen from others (and the bid calendar) so take my words with a grain of salt

you have to bid your karma, but if you don't win you get it back. I hope that helps