Definitely worth reloading a 28ga. Factory rounds are pricey, when you can find them. I took a quick look (and I mean quick, through WSS) and found that only Winchester and Federal offer a 28ga field load. $24-$27 per box. I checked with my brother today, he buys cases of the cheapest 12ga field loads he can get each fall when CT has them on sale. Says he gets between 35 and 40 16ga rounds out of a 25rd box of 12ga. Last year he got them for $60 a case. That's cheap ammo. In your case, I'd venture you return would be well above 50 shells per box. At only the cost of some additional primers.