Cheapest way to get rid of aphids on plants?

Hose them off. A firm jet of water knocks most of them off and they rarely climb back. For the stubborn ones, a few drops of plain liquid soap in a spray bottle of water, sprayed on the undersides of the leaves where they actually sit. Do it in the evening, not in full sun, and repeat every few days for a couple of weeks because new ones hatch. If you start seeing ladybirds, ease off - they'll finish the job for free.