In your question, it sounds like you only want to see this concert live once, so your choice is between seeing it now “for free,” but sick, or later for $50. Th

In your question, it sounds like you only want to see this concert live once, so your choice is between seeing it now “for free,” but sick, or later for $50. That is not a sunk cost fallacy, since the current marginal cost is lower to see them now. But there are many factors that can be a sunk cost fallacy too. If you hope to go again anyways, for example, then that future interest is not a factor now. Or if you pay more than $50 extra (or refuse to pay less than $50 extra) to get to the show tonight. Since if you incur a new marginal cost by being sick, you must consider that cost in relation to the expected future cost of the show, not the money already invested in it. So to avoid the sunk cost fallacy, you should always be willing to pay an extra $50 (in expected total), even after spending multiples of that (unexpectedly), and then still not go if someone would charge an extra $51 unexpectedly.