"You spent how much on that cartridge?"

Should there be a ratio….

No, unless you want to drive yourself nuts judging your or other “ratios” which lead to nowhere.

+1 Hobbies are simply for personal enjoyment. 

Ratios are useful tools in finding relationships and forecasting future results. It is not useful in high-end audio as each individual has unique disposable incomes, unique audio budgets, and unique personal sonic preferences. 

Fascinating subject.  I am thinking about my VPI HW19 MKIII, bought new in 1986, but now fitted with an SME 3009 Series II Improved, which is even older.  Oh well, I am playing mostly old records anyway.  Having a lot of fun too.  As I sort thru these records and enter them into the Discogs database most of them aren't worth much either.  Maybe I should hang my head in shame.  I am also mostly using MM cartridges too.  Not much to brag about.

a formula is not a bad thing when upgrading and you are a beginner.

Luckily I never had the budget to have to limit my upgrade by a formula, I always went all out and spent 100s of dollars!

I am not a cartridge man but I suppose different music can sound best with different cartridges. 30 would be too many for me but five might not.

I had this kind of experience with RCA cables when I had solid state phono stage. Now with VAC's onboard MM phono stage I don't need additional cables at all. Nottingham Space Arm's wiring goes straight into the amp.