First of all, as was mentioned in an earlier answer - are you referring to a second mortgage on an existing home, or a new mortgage on a second home? Closing costs differ somewhat for the two.
There really is no formula such as percentage of sale. I will be happy to break down the closing costs for you once I know what type of mortgage we are talking about.
The very best way to do that is to ask any mortgage company or bank--all you have to do is tell them what your insurance, price of house, and taxes are on the property (if you know), and they have software in their computer to do it quickly and pretty accurately. Also, most realtors know how to do this as well, so you can ask a good realtor as well--it's that simple, and that quick--most professional mortgage officers and realtors are happy to assist you with such information.