In all seriousness, cheapest would be DIY: oil and filter + for long term, a wrench, a pan, and a funnel. Takes an hour max. And it is actually really easy–I’m a mechanical idiot and can handle this task. Or call WSHS; I understand the shop class works on cars for free or cheap, and I’ve met the shop teacher and totally would trust him to verify the job is done correctly (way more than those chain places).
If you don’t want to do it yourself (I didn’t) or can’t wait until school starts…Jiffy Lube is $42 (a little higher for synthetic oil) and a quick search will get you a $15 off coupon that you can print or show on your phone. It includes five quarts and checks on other fluids, so worth 27 bucks for me.