I agree. A practical method to limit the advantage of real $$ investment would be to limit the worlds. Kinda like what Runescape (Jagex) does - they have space that is limited to free players, or to the benefits a free player could have. You couldn't do exactly the same here - because this game is constructed differently, but I think you get the idea.
Maybe like this - Bohemia free only - no troop or supply expenditures allowed, silver/gold accts benefits limited to navigation-type benefits
Livonia $10 per month,
Beta $unlimited