i've seen them outside a 40th storey office building window, which would mean somethink like 600 feet - but there could have been updrafts, so the elevation may be fortuitous, rather than deliberate.
The Monarch butterfly is known to reach 11,000 feet when migrating. Observations of the Monarch have been made by glider pilots at this altitude. Airline pilots flying between 10,000 and 13,000 feet have also reported seeing them.