Take advantage of special promotional prices. Take advantage of bundles. Titles may be packaged together, which can be a great buy if you're interested in all of the titles in the group. Prices can vary greatly among retail outlets. Take advantage of educational discounts.
Many software publishers have special prices for educational and government entities.
Ask the school lab instructor, librarian, and other knowledgeable users about specific software that might fill your needs. Most distributors publish software catalogs that list software, often by topics or categories. These can be picked up at technology and computer fairs. Download demo copies from magazine or other web sites. Borrow a copy, if possible.