He is valuing the logs at $600. Around here firewood is going for around $100 per full cord, cut split, delivered and stacked. How big is the pile of logs? What kind of wood? Are the logs something that could be cut into boards or is it just suitable for firewood.
It can cost $2500 to have just one big tree removed around here, withe the cutters taking all the wood.
As for the large rocks being put in a pile. He had to do something with them. You may be able to sell them to a landscape or garden store, and get them to come and pick them up. You wont get much if anything for them, but they would be gone.
His bid does sound really low.
If there is a writen contract he is leagally bound to that price, unless there is fine print in the contract boilerplate that allows for the unexpected.
Like others have mentioned, consider the future. Many contractors know each other, word may get out that they need to bid high to work for you.
AS to how much if anything to give him, that is tough without being there.
Look at it the same as a tip that you might leave a waitress that is exceptional.