The question is, "how much more expensive", and the answer W.R.T. plain-vanilla LLCs where the transition to C-Corporation is uncontested by all the members and all the members are still active is, "not very".
I'm sure you can easily rack up more than $10k in conversion costs if you let things get complicated, but I'd like a cost breakdown or even just a citation to a source for the idea that LLCs add $10k to C-Corp costs in the common case.
If I get to the point of needing my LLC to become a C-corp, that $10k will be a drop in the ocean.