For the longest time I've been combining smaller enchantment books (e.g., sharpness I) into larger enchantment books to put on gear. I'm now in 1.12 and it doesn't seem to work anymore, it just becomes too expensive.
For my sword, I started out combining sharpness I books into II's and so on. Once I had done this for all the other sword enchantment books, I began putting them on, starting with the highest level enchantments. Right off the bat sharpness V alone cost 28 levels on a brand new sword, free of any enchantments. From there everything else was pretty much too expensive. I began to do some testing with anvils and apparently you can't combine low level books into higher books anymore because they'll just become too expensive when you're ready to put them all on your gear. Instead, you have to start out with high level books (e.g., sharpness IV) and then combine them as usual before putting them on your weapon or tool. Since when did this change and how do I go about maxing out my sword?