This information might be outdated but during my last trip to Nepal, I saw a sign at the airport that anything above $2000 should be declared at customs.  So, I guess you can take as much money as you want, but if you are to do the official way, you might have to pay customs/taxes/bribes etc.  You know how it is at the Kathmandu Airport, people over there are like parasites, waiting with their mouths wide open.