Most of the places that sell books for a penny charge a lot for postage, so that the books end up costing at least as much and sometimes more than those sold by booksellers who are honest about both the books' prices and the postage.
As far as I know, the penny price is to get a higher ranking when books are listed by price. It's a gimmick. Many of those sellers also charge postage for each book you buy, so you never get a volume discount on postage the way you do with less unscrupulous sellers.
Also, the autograph doesn't make the book as valuable as you might think. Condition matters, and demand. I don't know where you found the $800 figure, but I found signed copies of the same book on Abebooks for about $30.
Which is still a nice amount over one cent.
When checking out book prices online, one should never go with the highest prices asked. There are always people trying to sell books for far more than they are worth, sometimes because they are starry-eyed optimists who hope to make a bundle, sometimes because they are scam artists hoping to find a sucker.
Remember, just because a price is asked for something does not mean that anyone anywhere has ever paid it.
A good rule of thumb is that a book is worth close to the lowest price asked for it online. And even autographed editions may be more interesting than monetarily valuable.
As a (not autographed) example of the online versus real value of books, I have a complete set of the works of Balzac from about a hundred years ago, something like 52 volumes with nice engraved illustrations. Online the set is offered for about $400 and individual volumes run about $15 each, but as a used bookseller explained to me, almost no one reads Balzac these days and the books do not sell. Whatever the price asked, their practical value is zero. They are neat, but they are not valuable in the financial sense.
So enjoy your book, and enjoy the neatness of the autograph, but don't be too dazzled by its apparent value.