I can't provide information about the cheapest website to buy books online. Generally speaking, online bookstores would regularly launch promotions to lower the price of books. You can try to compare prices in different online bookstores or go to the local bookstore or library to borrow books. This can save money and enjoy reading.
There are quite a few. Big names like Webtoon, Comixology, and Marvel Digital Comics are some of the popular ones, but there are many smaller specialized sites as well.
Webtoon is popular. It offers a diverse collection of comics in various genres and has a user-friendly interface. Also, Marvel Unlimited is good if you're into superhero comics. It has a vast library of Marvel comics.
AbeBooks can also be a good option. It's an online marketplace that aggregates books from various independent sellers. You can often find used light novels at very low prices here. The sellers on AbeBooks set their own prices, so with a bit of searching, you can come across some great deals. However, be aware of the condition of the book and the seller's reputation.
Book Depository is also worth considering. It offers free worldwide delivery, which is a plus. They stock a large number of English novels, and it's very likely that you can find the 'Arslan Senki' English novel there.
Definitely not. Marvel and DC Comics have always remained distinct entities in the comic book industry. Each has its own unique characters, storylines, and fan bases, and there has been no buyout between the two.