Basing library budgets on output measures (such as borrowing statistics) is a strategy that has its flaws. Such data that is gleaned is only quantitative in measure and doesn’t offer any depth of analysis. For example, a book borrowed and taken home may not actually be read. Alternatively, a book may be read individually by a student during a quiet library reading session, but is not accounted for in any statistics. Also, a particular stage teacher will often borrow a resource, for it to then be shared amongst the other classroom teachers (therefore not reflecting actual usage in data).
The library budget is only one component of the overall school budget and there are many competing school priorities. This all has to be taken into consideration when school budgets are allocated. In this regard, it is advantageous that the teacher librarian first and foremost has a great relationship with the school principal and executive and secondly, be a part of the school budget committee, if possible. This helps the teacher librarian to be positioned in order to effectively advocate for the directing of funds to library priorities.
With regards to actual budget spending, it is best to have input from those using the resources, ie. teachers and students. In my own school, I have a physical suggestion box, where books can be suggested. I have also created a google docs form that can be filled out by teachers and students alike, to put forward their suggestions online.