How much should a lawyer charge for service in Nepal?

Am afraid there is no one easy quick answer to your question.
Besides, if you are located physically in the USA, and expecting a Nepali Attorney to email you anything that resembles a Legal Opinion, Attestation, Affidavit, my gut feeling tells me:
1. it's not do-able (I know my nepali lawyers would never agree to send an official anything just like that by email) - Besides, they'd probably want to meet/know you in person (in Kathmandu) prior to rendering that service, and receive the set mutually agreed remuneration right then and there.
2. To do it right, the opinion/statement/affidavit or whatever should also most probably best be set down on the nepali attorneys' letterhead paper, duly signed, and show official company Seal.
Hope you can manage it somehow and get what you want.

Yes, exactly what Francoise is saying. My lawyer would also not agree for doing this without knowing you personally already.
Not doable about sums it up.

But that was not your question...
I would expect to be paying between NPR. 5.000 and 10.000, depending on the type of letter. So, about US$ 50 to 100.