What is the minimum Cross AppDomain communication performance penalty?

If you count lines of IL involved in each scenario, you will see that the CLR is doing much more than 100 times the work when remoting. A direct invocation is just a few opcodes, but with remoting there are multiple classes involved, real/transparent proxies, security checks, serialization, yadda yadda yadda. You will need to address this through design - there is no magic bullet for improving perf through implementation.

Is there any way you can call a single helper method that takes parameters about how many times you want to call the method you need? Cross-AppDomain call performance varies greatly by implementation. I believe it could be significantly better in the CLR 4.0, but I'm not fully versed on the details there.

Generally though, you want to avoid the overhead by "batching" the calls through a helper method.