I'd say the scanning part is the easiest. It's pretty much a mindless, repetitive task. I usually listen to music or to an audio book/podcast and check it at the end, page by page, counting the even pages if the book is small enough to get two pages at once. This part is very important and you should definitely take a few minutes to do it.
For me, the other side of the coin - the processing part, takes much longer: proofreading it in ABBYY FineReader, tracking down fonts, vectorizing the cover and various other graphics, doing the layout and proofreading the final product again.
My advice is don't spend more than you have to. Today a scanner around $100 was considered professional-grade only a few years back.