One other thing, there are these ridiculous mega packs for 7500 gold. The amount of reward for gold (namely meaning credits for upgrading cards versus gold) seems good, but are ridiculous cost for real money purchase. However if you save gold for a while you can earn them through gameplay. So if you want bang for buck, saving all your gold for one of these mega packs, and not using gold to refresh missions can be worth it. The downside is using gold to refresh extra missions is a great way to speed up progression in the early game by getting you more credits to upgrade cards. Because credits are the bottleneck, not boosters.