I'm not sure what the pea gravel will do???? The road base, limestone or cleachie will lock together and give you a solid surface. Adding another layer on top of that just seems like something that will move around and either end up in the grass, or get mixed into the limestone. The only time I can see where peagravel is needed is a parking area or for cosmetic uses. On a driveway or road, it's just a waste of money.
A good rule of thumb for buying rock is that 25 tons will give you 100 feet of road four inches thick and about 10 feet wide. A belly dump or end dump will hold around 27 to 28 tons. A standard ten wheel dump truck will be around 10 to 12 tons. I paid $14 a ton two years ago for limestone, but really think it's gone up with the price of fuel, but don't know any actual numbers that are current.
With your type of soil, there's no reason to dig down any for the road. Just scrape off the grass and stary laying rock. Be sure to have the rock higher then the grass so that water will run off of it and into your drainage ditch. Roads can be fairly complicated, but in your case, it's really a simple job. Don't overthink it or waste allot of time, effort and money removing dirt that doesn't need to be removed.
Eddie