Personally, I think the point of diminishing returns effects the turntable before it does the cartridge.
I agree, just don't put a $5000 cart on a $300 table.
Cartridge to turntable price ratio
How much do you spend on a cartridge before you have more cartridge than turntable. For example if you spend $2500 on a turntable and tonearm how much maximum would you spend on a cartridge? $1000? 1500?