This will depend on how much you plan on putting down and what type of interest you qualify for.
To get a rough estimate of your monthly costs you can use a mortgage calculator:
Then take the taxes of the property in question (which are available with the listing. If it's a FSBO, you can research the property on your local treasurer's website) and divide by 12, plus a safe average for homeowner's insurance for the type of home you are considering and divide by 12 (again, this varies far too much to give you an average here. You would be best off consulting an insurance agent to get a quote). Finally, to determine utilities, your Realtor can inquire on your behalf the average monthly heating/cooling bills for the property in question.
Add it all together and voila! Your estimated monthly operating costs.
Sorry to be so vague, but if you can fill in the blanks, this is how you can determine a pretty good rough estimate.