New costs of operations are all over the place. Some became very cheap, some really costly. When it comes to custom_json, based on historical transactions the cost went slightly up, however once the optimized parser is factored in, it might actually go down.
There is new mechanism that measures resource popularity in time window (currently a day), so when you try to do the same as everybody else (f.e. play Splinterlands), the cost might still go up, however if in the same time you try to do something else (f.e. write a comment) the cost might go down, because the state resource that comments require won't be as popular at that moment.
The biggest unknown is change in user behavior. HF26 introduces RC delegation mechanism, which might free up a lot of RC that so far was unused or burned on free account tokens. More used RC means higher level of resource consumption which rises RC cost of everything.