This is not really an answer to your question, but rather a discussion point. Why is it important to know the cost (in terms of execution time, I assume?) of using Elmah?
This might be a case of premature optimization. A hundred milliseconds here or there shouldn't matter, unless within a tight, long loop. If your application throws enough errors that Elmah's added cost accumulates to the point of dragging performance down, it is my opinion that you should instead be looking at WHY so many exceptions are being thrown.
Is it to defend its use to your peers? Most logging blocks will exhibit very similar performance characteristics, one of the biggest factors being the location or nature of the log destination. I/O operations like writing logs to disk or to a network destination will take far more time than the rest of the logging code.