How much? The spaces are $1/day, & you can claim it for as many days as you like—up to 30.
How big? The ads are sized at 150px by 125px. See sample:
TO PLACE AN AD:
1. Email your 150px by 125px ad AND dates you want to advertise, to:
2. You will be invoiced within 48 hours.
3. Pay invoice within 48 hours.