If you book a reserved instance, the price for lnx get as low as $0.01 ($54/yr). It's a bit premature but for spot instances, atm windows ones are around $0.0135 (linux history is not yet available). As for other instance-types it looks like that with spot instances you'll get the usual 60% off the original price.

EDIT: $54 upfront and then $0.01