Are journal back covers, inside covers (i.e. not front covers) worth the cost?
I would assume your question can not be answered without a personal perspective - what makes it "worth the cost"? Someone liking it? Two more people reading your paper? An invitation as invited speaker? 2 million [insert a currency of your choice] of additional funding per year? 

To be a bit more serious: I would check how the pages are included in the digital version of the journal. Most readers do not read the journal but go to the website and pick specific articles. If the images are not shown, very little people will notice them.