Obviously something here has to change, although it’s hard to know what without more details.
On the ads front, you may want to try going after more niche keywords with less competition to see if you can hit a lower average cost per click and/or a more informed audience. You should also make sure your ad copy does a really good job setting the customers up for what they’ll see when they hit your landing page. Some people just don’t read, but you’ll save money if people who don’t want your product can figure that out before clicking.
I have a hard time believing that there is literally nothing that you could do to improve your website. Do you have reviews from happy customers? Do you clearly explain the benefits and features of the product? Do you have an amazing return policy/customer service/whatever that sets you apart from your competitors? How is the checkout process? What about on mobile? Do you do any up- or cross-selling?
On the customer side, what is the lifetime value of a customer? Spending $25 now on a customer who will spend $80 over the next year is almost definitely worth it. You may also want to consider increasing your prices slightly, or only offering free shipping on a minimum order size. These things may lower your conversion rate (or maybe not!), but it might be a worthwhile trade.