Animus Books

so i had like 1700 Soldiery and brought my second Book for my Ninja Animus the other day was Sunday when i got my second book and on Monday i was capping my Poetics with a friend and saw i only had 800 Soldiery left i did 2 Sastasha HM to get the extra Soldiery and its after those 2 runs i noticed i had alot less than i should have, it looks like the book cost me 1000 Soldiery instead of the 500 cause i havent brought anything but that book, from what i can tell is buying books is costing more me than it should.