You can get the "X-Men Endangered Species" for $8 (hardcover $15) on the amazon.com, and with all the additional costs, you can get it even for around $12, depending on the shipping options you'll choose.
About the prices themselves: trade paperbacks(issues forming one story arc collected into one book), are usually between $10-$30 - price depends on things like: length, publisher, amount of the additional content and type of the cover (hard/soft)etc.
Single issues can cost from $1.99 to $5, depending on publisher and length, but majority costs around $3 or $4.
Personally, I tend to buy trades and I mostly read books from image and "Locke & Key" from IDW. I am reluctant to pick books from DC after the "relaunch" and cancellation of their only books I regularly red: "Doom Patrol" and "Secret Six". From marvel I usually end up picking "the Amazing Spider-Man", "Thunderbolts" and occasionally some "Deadpool."