@Boltgreywing
If the primary goal of the tax is to remove money from the economy it makes more sense to tax rich players the same as the poor (if not more).
Yes, quantitatively the rich will be paying much more in a 30% tax rate than the poor (the number will be bigger...), but qualitatively the amount is equal (you are taking away the same percentage of their buying power each way which is inherently fair).
Furthermore, generally those who have a lot of money have an easier time making more since they have more influence, access to resources, and capital to invest. Poorer players generally will not have those resources or much capital they can invest in attempts to earn more money so every penny is harder to come by for them and being disadvantaged by a tax rate skewed in the way you propose would be frustrating. Essentially, at the same tax rate a rich player will pay a much larger amount, but the poorer player will often still 'feel' the tax more even though they pay less.
Also, on a similar note to what
@Anoua was saying I think there needs to be some 'fair excuse' for such a tax. A lot of games have a small trading post tax which is rather unintrusive because it is in exchange for a service (essentially it is purchasing the right to list a trade/auction). A flat annual tax not tied into some gameplay feature or event could ruffle some feathers in my opinion though (or in the very least trigger some very clever but undesirable tax evasion attempts among your playerbase). It will seem like you're just taking their money for no reason. It's generally better to set up your sinks in a way that feels fair and voluntary like allowing players to spend money on some kind of upgrade (like lair expansions in Flight Rising), adding expensive cosmetic items for them to purchase and collect, or exacting a fee for certain common game activities (like the trading post fee I mentioned).
Those are just my thoughts though.