These are just my opinions, i.e. what's happened to me. YMMV.
WOM Cheapest, and the best overall, DEPENDING ON WHICH CUSTOMER REFERS YOU, AND WHAT KIND OF CUSTOMER THEY REFER YOU TO. Have had a lot of customers refer me to bad new customers; even a family member did that to me!
Yellow Pages. Best bang for the buck, paid advertising wise. It's where I get most of my new customers.
Vehicle lettering. Has gotten me looky-lous, but not much else. Plus, the damned magnets stuck to my car (had never happened before), and cost me $100 to have a body shop buff out the paint. Probably necessary to have them, but to me, a complete waste.
Radio Expensive. I was on-air for $50/wk, for a minute a week on a morning show, talking tech. Yeah, ROI here was two total calls, nearly six months after I was on-air. About worthless.
Newspaper Media here is a monopoly, with one company virtually owning every media outlet, therefore their paper is godawful expensive. Whereas a typical dinky classified ad might run ya $20-25/wk in most places, here it's about $80/wk! And no other local trader papers; the newspaper bought all of those up. ROI poor here as well.
Of note, I'd do Craigslist, but the closest town with a Craigslist to me is about 120 miles away. Doubt if folks will pay for the gas for those trips.