A lot depends on start or BIN price once you are paying for non store listings, it is only 50c for items under $20, from $20 to $99.99 it is $1.50 and $3.00 for over that amount.

If these numbers are correct, and I had items $100 or more to sell, I would be spending a maximum of $90 for 30 listings.

It costs $50 for a store. If you've got such high-end items to sell, open a store. If you don't, then you're only paying $15 more a month to list so, again you're making a profit. 

If you're selling your items at such low prices that you can't make the costs, raise your prices.

If you want to keep your prices low, find somewhere else to sell your goods and effectively pay more in storage because it will take a looooong time to sell them.