eBay has a tool called Fee Illustrator, where you can input your average number of listings, sales, and sale price, and it will work out the fees you'll pay for each available option (no store, basic store, featured store and anchor store).

You can also use it to find out how many listings / sales you'd need before paying for the store becomes worthwhile in the sense of saving you money, so inputting current stats as well as projected ones should give you a decent idea of when it will be best to upgrade. 

https://www.subs.ebay.com.au/feeweb/feeillustrator