I've seen these go away on ebay for €125 in (I believe) 'like new' and 'very good' condition. Even unpunched, just the 3 tiles and the rules.
I have the expansion and I must say, it's not worth the cardboard it was printed on. It is easily the worst Carcassonne expansion. And I include The Catapult in that list. I'm not getting rid of it, but I also am glad I don't have to contemplate spending €125 on it. It's just not worth it.
While I understand this community view, I do not necessarily share it. I do not have such harsh feelings towards Carcassonne Darmstadt, and do not consider it to be the "worst". The last word is in quotation marks, because many things can hide behind it. I know that people don't like Darmstadt because it is a small 3 tile expansion, inconsequential with very mundane rules, providing little benefit. On top of that it is one of the most expensive items on the list.
Let me tackle the price first - originally this item was quite cheap, in fact the entire sheet it came in (containing 5 different expansions) was very cheap. It is only because of the second hand market for the Carcassonne component (Darmstadt) and its collectible nature, that the price is so high today. You can still see other items from that sheet selling for relatively small prices (the highest being Catan Wedding tower for 19.49 eur).
What I want to say is that it should come as no surprise that a miniexpansion in old artwork with limited distribution would eventually reach such monetary heights.
Secondly the rules - people say this has uninteresting rules for a high price item, but you have to remember, nobody was expecting 3 tile expansion to have a huge impact in the first place. It was a meant to be a simple promotional item with just a little twist to spice it up. There are however many minor and even major expansions which promised something great, made themselves look promising and then did not deliver - sometime making them nigh unplayable (Cult, Tunnels). So the "disspointment" I think is far far worse for an expansion than a lack of interesting content, which has been forseen in advance. Catapult is silly yes, but it is meant to be silly and it made itself clear about it. Tower on the other hand promised to "take building in Carcassonne to the third dimension", but delivered... taking prisoners and selling them for ransom?? Right after another aggression-oriented expansion?
Lastly the artwork - it is beautiful! Why are people saying it is "low quality"? Is it because of the Darmstadium being entirely modern building? (that is not exactly true by the way - there is a reason for its inclussion).
So is Darmstadt the worst expansion? No. Is the high price of Darmstadt unfortunate? Yes. But it is a direct consequence of secondary effects known to occur in similar Carcassonne items.