Not to sound snarky, but you get what you pay for. If there is a company out there that charges $20 for the same book (size and pages) that another company charges $50 for, there is probably a reason. You also want to compare apples to apples.
I've gone to all these sites. Some of the pricing you listed weren't for the same types of books at the different sites. So I'm going to compare same sizes in the hardcover option. All books come with 20 pages.
For an 8.5x11 book:
Lulu - $25.95 for 20 pages, $0.50 for each additional page. This is a personalized cover.
MyPublisher - $29.80 for 20 pages with up to 240 photos, $0.99 each additional page. This is for a linen cover book.
CM - $39.95 for 20 pages in a linen cover book. $49.95 for 20 pages in a personalized cover book. $2.50 2 additional pages.
Disney Photopass - $49.95 for a personalized book with 20 pages. Limited to 80 photos from your own camera. $69.95 for a book with all that plus you can use your PhotoPass photos. $1.49 for each additional page.
Snapfish - $19.99 for a 20 page linen book. $29.99 for a 20 page custom cover book. $1.99 for each 2 additional pages.
Shutterfly - $29.99 for a 20 page linen book. $1.00 for each additional pages.
With most of the companies you are limited to the selection of templates and themes they have available. You are limited to the amount of text and photos you can use. It can get frustrating trying to fit pictures in where you want them but they aren't the right shape or size. If you can use a program like PSE or something similar, then that's not an issue. The Creative Memories program is a little more flexible than the others as well. I believe if you purchase the software, you are able to customize your pages with as many photos and text as you desire. Someone correct me if I'm wrong.
I know for a fact that Shutterfly's books are glued. I believe that other than Creative Memories, the others are as well. I am not positive about Lulu as I couldn't find anything easily on their site. **Note if it says perfect binding, that is glued.
So there you have an apples to apples comparison. Some of the sites offer other book sizes (8x8, 12x12, 5x5, 7x5 and more) with different options (dust jackets, paper books, etc). A few didn't offer those sizes so I had to use the more common size. The 8x8s are less expensive than the 8.5x11 on all the sites that offer that size. It's all going to be in what you want - quality, options, and service.
Hope this helps you.