Amateur astronomy can be as expensive as you want to make it. It is, however, ERE compatible in the sense that while asset costs can be high, the running costs are non-existent. Once you have the equipment, there's no upkeep.
Amateur astronomy is somewhat unique in the sense that amateurs can make worthwhile contributions to the field. There are at least two areas where the amateur can contribute scientifically. This is because the night sky is generally so vast that large telescopes and the limited number of professional astronomers can not possible cover all of the sky all of the time.
Variable star observation
This involves recording the magnitude of variable stars, typically by comparing their brightness to neighboring stars. See AAVSO for details.
Fun fact: Jacob/ERE used to do theoretical work on dwarf novae (a type of variable star).
This can be serious business but if you're willing to dedicate your life to it, chances are that you may get a newly discovered comet named after you.