I think the intent is to provide players with a sense of pride and accomplishment for unlocking different engrams. As for cost, they selected initial values based upon data from the Osiris DLC and other adjustments made to milestone rewards before launch. Among other things, they looked at average per-player engram earn rates on a daily basis, and they will be making constant adjustments to ensure that players have challenges that are compelling, rewarding, and of course attainable via gameplay. They appreciate the candid feedback, and the passion the community has put forth around the current topics here on the internet, our forums and across numerous social media outlets. Bungie will continue to make changes and monitor community feedback and update everyone as soon and as often as they can.