What is the performance impact of queries against DM objects using DMVs in SQL Server 2008?
There are other DMVs that can help you quantify this.  sys.dm_exec_sessions has columns cpu_time, memory_usage and total_scheduled_time amongst others.  While you're running your plan-getting query in one session you can interrogate this DMV in a second session to find how expensive the first is.

Pleasingly, you can also quantify how expensive your second session is at the same time!