Based of other games that sell skins the prices seem reasonable
400 gems = $5 usd
800 gems = $10 usd
Look at games like FF14 and WoW that charge $25-$30 dollars for a mount and no skins.
Its what makes GW2 amazing you get a raptor you get a raptor you get a raptor. How/if you want to make that raptor look entirely up to you. The skin does not make it do anything different.
It's a business, they need to make money some how as they effectively just give away their content for free and the fact that you can grind away and buy gems with gold + all the other methods such as keys, I see no issue with the current prices, surprised they're not higher.