It can be, if you niche down. Generally, you are going to spend a lot if you target keywords like "Halloween Book," but if your book is specifically about a kid who is making a Jack O Lantern, target some things like "Jack O Lantern Book." Of course, you won't get the volume, but you will get a better conversion rate. Through the last 6 months of Amazon ads I have sold over $1MM that can be attributed to ad clicks and have spent less than $30k.