The more income you have, the more upkeep you will be allowed to have. You may use up to 100% of your income for your upkeep; however, you may not go above it. It is wise to use only 20-25% of your income for upkeep. This allows you to arm up all or part of your alliance while still building your income for the future.
Your empire dictates the amount of income you are receiving hourly. More bases = more income.
Properties and land are not the only way to increase your income. Many people will rely heavily on collecting bounties to build their empire. This is one of the fastest ways to increase your income, but also the hardest. Missions can be one of the slowest ways of building your income.
How good is your income? - If you have the maximum amount of allies for your level and you can equip them all with the strongest available henchmen and vehicles and use only 25% of your income or less, then you have a great income. - If you can arm up all of your alliance members with a henchman and vehicle, but it uses 50-100% of your income, then you have some building to do, try claiming some bounties before you level up. - If you cannot arm up all of your alliance members because you would be using more than 100% of your income, then you should definitely be considering staying at your level and building your income.