For London it's ?2 a pop to move yourself to the top of the listing, so costs depend on how many times a day you want to make yourself more visible. I sometimes bump 5 or six times, and don't necessarily get a booking. You can also purchase a little "bundle" of 4, 8 or 12 inserts, but they will be posted on the hour, together will lots of other ads, so you drop out of sight again almost immediately.