How much should a local service business realistically spend on seo?

I run a small plumbing biz.

Started SEO at like $800 a month, then cranked it up to $2k–$3k once leads started rolling in. Used an agency to figure out what each lead was actually costing me.

Now I’m just trying to keep it under $1,500 a month and still get steady calls.

Anyone else here running something like plumbing? What are you dropping on marketing right now?