Although your profit is a bit low- you made money on your first job! NICE!
If you are uncertain about the prices of stuff- dont be afraid to step down and tell the customer that you need to "shop around to see who has the best product and you will get back to them asap." This gives you time to call around and see what your place is selling them for- and a good time for you to sit down and think about what you can do the job for while remembering the fact that you will need to buy a new truck, dump truck, and mowers of your pleasing every ten years.
Ive learned to never shoot from the hip- much rather look like an idiot than make an informed decision rather than getting burned.
Personally I would have doubled the cost of the materials.