If the rumours are true that chelsea would be owed 2.5m per game, it makes absolutely no sense (financially) to play courtois. Contrary to what many fans may think, the benefits of having courtois (or any other starting 11 player for that matter) over the 2nd choice is marginal. But 2.5m is a lot of money to anyone.
ie. the betting line for this game wouldn't move that much based on whether Courtois will or will not play.
And yes the club have probably made a lot of money so far from their ucl campaign, but that doesn't mean that they can/should now throw it away.
I think all semi-finalists are garunteed 5m euros regardless of whether they win or lose. And you get 6m if you lose the final, and 10m if you win. i think the prize money for the final would have to be around 60m and 100m before you can consider actually playing courtois at 2.5m per game in the semis.
Surely a simple official letter to Chelsea saying they can't afford the fee can he still pay would work. As Chelsea wouldn't be able to refuse the request and then they wouldn't be able to request the money. Just depends if Atletico want to keep a good relationship with Chelsea and potentially keep the player for next season.