Always get lost when it comes to pricing 'rock walls' using boulders.
I know dry laid stone walls are in the range 450-500 a pallet, which is around 40-50 a face foot.
Being the material is 'free', I would say a good start is around $20 a face foot, but its hard to say. Have no idea what kind of wall you are builiding. If you are just 'slapping' rocks up, then maybe less. If you are going for a 'tight' look, filling in the voids, placing each rock carefully, I'd say a lot more.
If in doubt, I'd price it out on a 'per day' basis and cost of materials/cost of acquiring materials. IE. if you are going to be there 2 days with the machines and 2 operators, then bill out the daily rate for the machines/operators, then add in the cost of trucking in the stone, then add in all the other stuff. At least this way, you know you are covered.....but need a good idea on how long it will take.