I also have an air fern. Currently he is on the Nutrena Balancer for grass hay/forage and he gets less than 1 pound a day.
More like 6 oz in the AM with his hoof supp. and a tablespoon of whole flaxseed.
Pm feed is about 4 or 5 oz with no supps.
Very average hay and forage when there is some and he is sleek (well soft and fuzzy right now) and rolly polly.
(He's in my barn here- Name's Digby and he has put on weight since I bought him and brought him home from KY.)
I've also used the Buckeye Grow N Win RB and liked it. Slightly cheaper than the Nutrina RB, but my supplier was having ussues.
Anyway, one bag of horse "feed" lasts me two months at a time and all told I think it costs me maybe 0.75 cents a day to feed him, not including hay.
I do have out a mineral block and I did run all the data through FeedXL to make sure I wasn't missing something and he scored nearly perfect.
For air ferns, RBs are the way to go.