Real-life MMD: Should my replacement on theatre trip contribute to ticket cost?

If you were asking for your ticket to be offered/sold at a discounted price in your absence, you should have made that clear beforehand. If nothing has been offered by the lucky recipient, then (although-personally I think like you, it's bad manners), you have lost out.

The onus was on you to make clear at the point you offered the tickets up whether you expected some/part payment.

I recently had two panto tickets which I couldn't use so offered them to a friend, making it clear that I didn't want any money, I just didn't want the tickets to go to waste.

With last-minute cancellations, I think most people take the view that it would be a shame for the ticket to go to waste and, since the money's lost anyway, someone might as well use it.

In my office, people quite frequently email around last-minute offers of tickets that they can't use, and almost always specify that the tickets are free to anyone who wants them. However, if they didn't specify, then I'd always ask whether they wanted any money for them rather than assuming that they were free.

In this scenario, I think you should have broached the money at the time if you were wanting to be reimbursed, rather than just expecting that it would be offered. Even though I think it's rude for the recipient of the ticket not to ask at the time, I think it's too late for you to ask for money now. After all, the replacement may only have agreed to go along because they were under the impression that ticket was free, and if they'd known that they were going to have to pay then they might not have been interested.

This situation is further complicated by the fact that you got your friend, who was organising the whole thing, to find a replacement instead of finding your own. Speaking from personal experience, organising a group theatre trip can be incredibly stressful, and I'm sure that running around trying to find a last minute taker for your ticket wasn't particularly fun for her, so I think it would be a little unreasonable for you to then bring up the subject of money after the fact.

Manners and decorum can no longer be relied upon nowadays I'm afraid, and there is always somebody ready to take advantage of someone else's misfortune without as much as a twinge of guilt.

As has already been said, you shouldn't have had to, but in 20/20 hindsight perhaps it would have been wise to have made it clear to your friend on what terms the ticket was to be offered up, rather than to assume that she would not have given it away for nothing.