How to find prices for bulk amounts of various materials?

I am constantly developing a range of ideas involving large quantities of materials. My creativity is often limited because I do not have any clue the prices of large quantities of materials. Right now I for example need to know what the price of 1 million kubic meters of asphalt would be. The price I get might be 30-40 % wrong, but then I could continue with estimating whether my idea is just way off or not. There are several problems with obtaining such information from suppliers as they speculate in getting the highest price and therefore would not risk saying a price that is 40% too low, as they would never be able to negotiate the price upwards later.

I would need the prices for the following excluding transportation costs:
- Cement (any fairly normal type)
- Clay (used to make bricks)
- Sand (any fairly normal type)

Where could I get such pricing information?

You assumption maybe wrong that there exist such ting as "price of 1 million cubic meters of asphalt". The price is what you suppliers offer to you. This is the rules of the free market.
Prices for such tings are location and season dependent. There is also international exchanges for some raw materials.

Look in industry publications for construction. ENR (engineering news record) comes to mind. There are tons of magazines and publications that follow construction material pricing trends, steel, wood, asphalt, cement, etc.

Also, the price for 1 million cubic meters of asphalt is going to be dramatically different than if you bought, let's say a few hundred or a few thousand cubic meters. Asphaltic Concrete (AC Pavement) is sold by weight, delivered and installed. The companies that sell it mix it in house - gravel / sand / tar (asphalt).

So, to release your creativity, look at commodity prices, industry publications, ask your local suppliers (yes, pick up the phone and call) and update your data frequently - these commodities fluctuate a LOT.

Good luck!