The word I would use is solvent, though this is perhaps a slight stretch of its dictionary definition.
Oxford dictionaries online defines solvent as:
Having assets in excess of liabilities; able to pay one’s debts.
By this definition one could be "solvent" if one is steadily losing money but still has a large amount of capital remaining.
Defining it as "Income greater than outgo" gives a more practical target to work for in personal finance or business finance. Also this is not out of line with the word's etymology as "loosening, paying out."
(Note that unlike "frugal" which carries connotations of sensible or intelligent and conservative use of money, "solvent" has no such connotation and merely reflects the numerical fact—whether that your liabilities are less than your assets, or that your income is greater than your outgo. You can spend money on wasteful whims and remain solvent if your income is sufficient, but the word "frugal" would no longer describe you.)