Hi,
Indeed, that seems like a normal fare for non-residents from Lan and Avianca. The two other domestic airlines, Peruvian and Star, do not make a distinction between residents and non-residents, so you might find better fares with them.
A very quick search on Star Peru website gives me $227.26 for july 28 - august 2nd. I guess it's possible to find a bit better changing the dates, but that should give you an idea.