How come nobody has asked "What does the application do?" - this is fundamental!
If it's a front-end appliction, using screens (CICS or MVC) communicating with COBOL programs, there's no doubt it will be easier to maintain in C#, and will probably look a hell of a lot better too. However, it's a big migration from one to the other.
On the other hand, if it's a big batch system designed to process thousands of records, stored in files in fixed-width format, this will be a lot trickier to do in C# (I don't know C#, I'm assuming it's possible, but I think it's probably trickier).
Your cost-benefit should look at the the purpose on the migrated/amended/rewritten system, and decide on the best technology to get the job done.