I agree - always been a bit mystified when fusion power is described as "cheap", fuel isn't a big component of the cost of fission power plants so even if the fuel is low cost it won't save you that much money.
[I guess decommissioning costs might be lower but are they actually included in the costs of operating normal nuclear plants?]
I bet regulatory compliance is a big chunk of the costs. Those regulations are bound to be much more strict for a fission plant that can potentially blow up and contaminate a large area compared to a fusion plant with a safe failure mode by default (the plasma just shuts down).
The worst case scenarios for an accident at a fusion plant aren't nearly as bad as those at a fission plant so you should be able to get by with somewhat less in terms of paranoid safety measures and overbuilding.
[I guess decommissioning costs might be lower but are they actually included in the costs of operating normal nuclear plants?]