Replying to [davindo]:
What germex said has expressing the "big picture" of doing business which is getting the highest profit with the lowest cost.
Pre calculation for a time framed forecast will be significant. in other word there would be differences in value between a 10 years supply and 2 times purchasing only.
Quoting what Lindy said "cheaper when direct buy". she's right. but this is not always be true. I can say this by looking at the seller's side (the manufacturer). When the time he had an agent which looking profit from sales value portion, manufacturer will decrease his profit. as the benefit from they have widen sales market (what could be better than having high market share, 2 x 5 will not the same as 10 x 1). Also with the agent. he won't sell higher then the released manufacturer price. Customers soon has a tendentious mind to gain more profit by buying directly if turn out to be more cheaper. There will be such agreement between manufacturer and their agents.
But, if we are going to deal in long term basis. we can use direct buy. a range of agreement should be made up with the manufacturer. a special pricing may occur. it may even lower than the agent's price. On top of all, use your due diligence.