You want to set the price for your product (sand and convenience) correctly.
The price for the sand is easy - it's exactly what the competition sells it for. Check around other locations,not just the one close by. Also see what volume purchase price is, not just small qty. Remember, sand is a commodity. It is not differentiated.
The price of convenience is a bit more complicated.
First off there is a 10 mile round trip. Figure 30 mph average speed and 3-4-5 mpg.
Travel time is 20 minutes. Figure 40 bucks an hour for the truck and driver is the cost for the company. 1/3 of 40 bucks is $13.33
Fuel cost 2-3 gal. Figure they have a large contract for fuel and it's at $2.75. Fuel is then $5.50 - 8.25
Using your competitor, if they can run for a load of sand, come back, dump and run out again all in 1 hour - then they will get 8 loads a shift per truck. If they get sand from you, they can do that in 40 minutes a load - meaning that they can dump 12 loads a shift. That's 50% more productivity. That's big money. It's bigger than the direct fuel/time savings. It's like them having 50% more dump trucks.
Your convenience is saving them 13.33 + 5.50 = $18.83 (up to $21.5. PLUS having the effect of making their dump trucks 50% more productive.
So if you charge a $18 per load premium, it would be attractive to them. It would probably still be ok at $25 per load, but might generate some hard feelings. Don't discount that! It may make you a target for a law suit just for "pay back". Human nature shows time and again that to be the case.
You do have a severe barrier to entry into this business. Namely, they can't trust that you are able to fill their trucks promptly. They don't have a feel for the quality of the sand either. Is there a premium for early hours, late hours, weekends, holidays?
You need to address those concerns when you submit your quote.
It may also be better to just charge for the sand at the same price as the competition - with the construction company doing the digging and loading. Less overhead, less time, much much less liability.