In-game purchases
With many other games, the real revenue comes from in-game purchases. From Smurf-berries to Pokemon backpacks. Another way to raise funds for TLK game development would be to sell other small advantages in-game in addition to the Duke accounts. The best example I can think of is selling a Healing Turn for £2, or 2 for £3. Available to Dukes and non-dukes. There would need to be limits so the game doesn't get out of balance and someone can pay to rank to marq in a day. Maybe 2-3 turns per age and not available until after 24 hours in to the game. There could be other minor advantages we can come up with- such as more gold. But the basic mechanics for getting HT's as a bonus are there for random joining, so perhaps that would be the easier one to start with.