You can find onigiri at independent shops not only convenience stores, with good price and better fillings/quantity. Convenience stores seem cheap compared with America but everything they sell is expensive locally
There are also onigiri adjacent foods worth exploring like tenmusu. I love this tiny place in Tsu (Mie prefecture capital) that serves 6 of them with soup for about $3, a full meal offered with dine-in, even though it’s now Michelin recommended. It's probably 1/3 the price of convenience store onigiri per unit cost and it's freshly made to order.
There are also onigiri adjacent foods worth exploring like tenmusu. I love this tiny place in Tsu (Mie prefecture capital) that serves 6 of them with soup for about $3, a full meal offered with dine-in, even though it’s now Michelin recommended. It's probably 1/3 the price of convenience store onigiri per unit cost and it's freshly made to order.