Fewer people doesn't necessarily mean that their costs have come down equally.
We know that as it currently stands, they are implementing extra safety procedures. More cleaning and stocking of sanitizer likely means more people on duty, which costs more. The convention center has also gone through upgrades, which I wouldn't be surprised might be passed onto those utilizing the space to some degree. I work for a real estate management firm, and believe me, if we upgrade a building, it's not done out of the goodness of our heart. Oh, the improvements may be legit good things, but that work comes at a premium as well, and it's not done entirely out of pocket with a smile.
In fact, with reduced admission (currently) lined up, people need to represent even more of the funding provided, individually. 1/2 attendance versus (hypothetically) 2/3 or 3/4 of the expenses is going to have to come from somewhere. Maybe that'll be tied to reduced profit margins, maybe it eats into some 'rainy day' funds, whatever it is, if the bump is only $3 per ticket, that's a rather unremarkable boost all told.
Plus, with the vaccine roll out and states acting in response to falling cases, demand might be high enough for there to be actual competition for slots. Just as Gencon missed out on 2020's revenue, so did the convention center, and they will presumably be looking to make up some of that shortfall as well.
It's a complicated issue with a lot of fees, costs, and contracts that we don't have access to (nor should we), but I wouldn't just assume 'smaller con means it should be cheaper'. If it was affordable in 2019, a minor rounding error in the ticket price upwards shouldn't be a deal breaker, and if it is, then hopefully those folks can save up and come back to a much more normal 2022 event.
Doubling the cost of the tickets would be 'sticking it to the customer'. A $3 bump is making a mountain out of a molehill.