It's a subjective question based solely on one's own income and financial obligations. 

For example, a $100 bill is worth more in the eyes of a homeless man than it is to Donald Trump. A $500 limit card held by a college student making $20k/year is going to be more useful than it is to a professional making $80k/year. This is along the same lines as the 'what makes a card prime' question that comes up every month here.

Wallet: JP Morgan Select Visa Signature $9000 | Chase Sapphire Preferred $5000 | Chase Freedom Visa $2000 | Barclaycard Visa $3000 | Discover More $1500 | Capital One Platinum Mastercard $750 | Bank of America Cash Rewards Visa $500 | Walmart Store $950 | Amazon Store $5000 | AMEX Zync (AU) NPSL | Chase Total Checking | RIP: First Premier Mastercard