Now cheapest store fee is $49.95 per month. It means, if you sell 100 items you pay 50c per item for store fee on top of 20c insertion fee plus 7% FVF.

So if you sold $3 item in store you pay $0.91 (50c + 20c + 21c) for fee when you sell 100 items per month.

If you only sell 50 items per month you pay $1 store fee per item. So for $3 item you pay $1.41 ($1.00 + 20c + 21c) for fee.

Insertion fee can be reduced if you are selling multiple quantity. If you list quantity 10 and sell all your insertion fee per item will be reduced to 2c per item.

You need to consider all factors to consider if a store is for you or not.

I think at this stage if you are not a powerseller yet using 30 free listing is better option. Once you become a powerseller you will lose 30 free listing. So you can thin about a store because you need to pay insertion fee 50 per item to list.