1. No it shouldn't be that high. 2. I havent actually heard of an OEM Honda alarm. Most of the time dealers pay a company to do it (which is usually not a good installer or install techniques). 3. aftermarket is ABSOLUTELY cheaper. 4. don't let the dealer state that it will void your warranty, that's completely false - read magnuson-moss act.
A customer of mine was quoted the same at her honda dealer. The honda remote start is stupid expensive when it doesn't even have the range as an aftermarket one. I would recommend saving some money and going aftermarket.