I do not like rule of thumbs but you may be able to try 3%. This amount is really to subjective since we do not know the particulars of your purchase.
First, start with an amortization calculator. Just type amortization calculator into in search engine and you will get a billion hits. This will help you calculate principal and interest over the term of the note.
Next you need to specifcially identify, what is the annual insurance, HOA fees, mortgage insurance and any other mandatory fees associated with this loan and property to estimate monthly payments.
In terms of what the loan will cost, again, each bank/lender is different for what they charge. I will say consumer beware of baiting and switching. Use a reputable lender/bank who doesn't bait in switch.