The minimum for a reward seems to be the cost of a commodity deed box or a crystal portal, both of which are year 1 rewards.
Given that both seem quite useful, many people will choose these, even up to year 9, because of their ease to sell. So you would likely be able to sell rewards for at least 4 mil.
I'm after a bunch of year 3 rewards (potted cactuses) personally and am paying 5 mil each or so. Year 6, however, you can get braziers for which are also quite popular (at least, for me
)