Fixed and variable costs
These are pretty easy concepts to grasp. The fixed cost is the amount you pay regardless of usage, and the variable cost, as the name suggests, varies with usage. A trap to be aware of is licences. I have come across schemes that will provide you with a lot of stuff for very little cost, and a low licence fee per user; PER USER. Bear in mind that a fee of £1 per user per year works out to £1,000 per year for a school with 1,000 users. Thus licence fee bands are probably better value.
Even better is what I was able to negotiate once, which was the software bundled in with the computers for no extra or ongoing cost. These days, of course, there are plenty of office applications which are open source and involve no monetary cost at all. Your variable costs in such cases are likely to be printing costs and, possibly, consumables like USB sticks. The main cost will be the initial outlay, ie fixed costs.
Another trap to avoid is buying lots of single-use programs — the costs soon mount up!