Halo Veteran fanbase seems to have a love for only grinding their hinds off for anything. So why not let the challenges also reward in-game currency, but the prices to buy cosmetics be really high? People will grind the challenges, get currency, and eventually get what they want and be satisfied because they got the armor it took all week(s) for them to get. Of course, the prices would vary depending on what the item they want is.
And if nobody wants to wait and grind, they can always pay that $10-$15 dollars to get the piece quickly.