best way to get people to back more is to offer single cosmetics. Once someone buys a 200$ pack i doubt they'll spend more when the next pack might be 400$.
however, if pets, skill animations, emotes, or other cosmetics were up for sale then it could persuade people to toss them an extra 5-15$. I believe skins for items, mounts, pets, skills, etc will be added so those could be sold too. I'd highly suggest having seasonal variation which are only sold during that time or are specific to the year and season being sold. I would offer that after a set amount of time those items could be put back on sale for a limited duration (optional of course).
DS could issue dynamight coins if a player buys something but they don't like it (instead of issuing a refund). IE you see concept art for a pyro blast, you buy it for 1$, they make it a few months later but you don't like it, you could refund the item for dynamight coins to buy something else at a later time.
@Prometheus i'm sure you know this but eventually people will get to a point where they wont buy another backer pack.