There are lots of things involved with the expense of any item, and that includes rock. I’m not familiar with the accounting end, but asphalt uses river rock. Concrete requires gravel, and gravel involves a lot more hours. It has to be mined, but also cut down into bits, and sized. Yes, sized. There are different sizes for various uses. One of the jobs I did with the DOT was aggregate and concrete testing. It was a process. Samples of gravel and sand had to be retrieved from the various supplying quarries. Each sample had to be washed, and not with tap water, which would affect the outcome.The runoff had to be kept because even the percentage of dust must be calculated. After washing, the sample must be heated, but not too hot, or quickly, because steam will pop big pieces into smaller pieces and ruin the results.
When the sample is cooled to room temp, it is dropped through a tower of stacked screens. Each screen is a different size, largest at the top. They are fitted in wood frames which contains the gravel of each descending size. Each size must then be weighed, and the dust also, accounting for the total weight, and weight of each size. This is a also done with sand but in a screen tower fitted for much finer aggregate, and that tower is shaken by a machine similar to paint mixers. All that cooking and shaking comes from science which has determined the best aggregate size percentage for various uses.
Each quarry also tests their aggregate with the same process to ensure the quality of any given stockpile.
All that involves man-hours. River rock is already precut.
I also tested concrete on site using simple tools to dump a sample into a stack. When the form is removed, the height is measured to determine the amount of slump which occurs, revealing the moisture content, and whether it is too much or too little.
One more test involved packing a sample into a metal box, and curing the proper amount of time. I then removed it from the box and loaded it into a machine to apply gradual pressure, and marking the PSI applied at the moment it breaks. Tim the tool man, whoa ho ho.
Asphalt only needs to cool, concrete must cure, which takes significantly longer. Deep south experiences a much lesser change in temperature and humidity, so they can get away with fewer repairs. Further north, concrete is fine for a base, but without asphalt on top, every crack would be a potential blowout for our tires. A crack in concrete is much sharper than a crack in asphalt.
I feel like I just provided an entry for Wikipedia. I hope at least somebody now understands.