The answer has everything to do with your book's sales. A professionally designed book is far more persuasive in the marketplace. In an attempt to save money, you can lose future earnings as well as the actual investment in time and money.
Many reviewers scoff at such a product and refuse to review it.
Distributors won't carry it if it is not commercial. And, retailers surely won't buy it. We understand and appreciate the need to save money in production. That is why we have priced our book design and production services so affordably. It is our desire to make this professional service feasible for self-publishers.