500 Gil -> 1 MGP
1 Billion Gil -> 2 Million MGP

They would have to increase the amount of new valuable items in the gold saucer each patch release.(Materials for a gold trimmed housing exterior that you can't sell, new mounts, minions, etc)
Downside is it would encourage new people to buy gil from gil sellers just for gold saucer rewards.

I'm surprised SE charged real money for the gold leaf flooring and wallpaper instead of a huge amount of gil since its literally gold interior.