More like a tithe, probably a fixed percentage of the member world's Gross Domestic Product. Say 10% Since the member states don't have to pay for defense or diplomatic functions (those are handled by the Consul-General and the local Count) plus they get other benefits from the Imperium, the money isn't missed too badly.

Worlds with tiny economic sectors might have GDPs so low that collecting the Imperial tithe would cost more than the actual income. In these cases, some symbolic gift is collected periodically.

Adventure hook: Umfrei, a primitive world home to a minor alien race has almost no real economy, so is exempt from the Imperial Tithe. However, the race has agreed to offer a bugaart, a local avian raptor, to the Count-Elector every three years for his personal zoo. To celebrate the first tithe, local artists crafted a magnificent bugaart of gold encrusted with jewels. Sadly, the pieces was lost to pirates. If found, it could be worth millions.

Can you follow the clues and track down the legendary Umfrei Bugaart? :)