Regardless of what the agency provides, you really need to make sure you can answer that question. (And good SEO people should be able to show you those results)
On-page optimizations - have you got the before/after metrics?
Keyword analysis - have you got the click rates on the new / old keyword sets?
Content landing pages - do you know the click rates and monitor the contacts/signups coming in that way?
Backlinks - what's the traffic / result?
Maybe $6k brings you a good result? Maybe not? It's not too much if they bring you extra $1M in sales. It's too much if they don't make a difference. But only you can figure it out.
> more clearly than just the vanity metrics we see in Google Analytics?
Google analytics can be used for this. But you need to look at specific funnels, (https://support.google.com/analytics/answer/9327974?hl=en) submit specific click events to track, (https://support.google.com/analytics/answer/9322688?hl=en#zi...) etc. There's no magic tool that will make it easy though - you have to tag the pages / monitor what's happening properly. You can do it on your own too, but a good tool for exploration will help.