It would also be beneficial to ask your lender for a Good Faith Estimate (GFE). This document will itemize the fees and costs associated with the purchase. Along with the mortgage payment breakdown, you will see a closing cost portion that will include the pre-paids. If you are planning on asking the seller to cover your closing costs/ pre-paids, it is best to know the exact amount before the offer so that you do not leave money on the table :)
Closing costs are determined by the type of loan you will be using to purchase the home. Some loans require that the buyer bring a minimum amount. For example, a FHA loan requires the buyer to bring a minimum of 3.5% of the purchase price. As a rule of thumb, if I have a buyer using an FHA loan and they are needing closing cost assistance from the seller, I will ask for 3% of the purchase price from the seller to cover the rest of the closing costs for the buyer.