The challenge with discounts is ensuring you can get them.  For example - buy from the store, only standard discount is available; buy using a pre-existing contract or corporate agreement, only get specific discounts.   In addition, I have watched people assume they will get discounts, especially if they have already made one round of purchases, and be very disappointed. 
The best bet consistently seems to be: budget based on the list price in the store; document the expected or required ROI or business case justification, and send in some decent negotiators.