Alot of it would depend on the tiller. Figure how long it will take to use the tiller to do a smaller area and multiply it out. Theres really no way of knowing without know your numbers. Once you find out how long you know how to charge it. Add it extra time for those dang rocks. Or if you want to do this as a learning experience (I'm assuming your renting) ask the rental guys if you think it will take a day or two. Then just charge a full day labor plus rental.
I figure it would take a full day i have three guys working on it. The most efficient way of figuring price was to buy the seed, straw and fertilizer and double the price and charge a rate for the tractor but if another day is needed it won't be a problem because we have the tractor and do not have to rent nothing.