To calculate the cost per square meter, you need to know two parameters - the value of the property and total square footage. Divide the total cost by the number of meters in the apartment and get the value of one square meter.
If you don't have the slightest idea about how much it costs your property, engage in monitoring. For this you will need specialized Newspapers and magazines, as well as the sites of real estate agencies online. Similar to your search for properties on the specified parameters. View prices on a particular object, write down their plate (so it will be clearer). After a analyze run-up in prices, average. Further work with her. The principle is the same as in the first case: divide the resulting value by the square footage of your apartment.
If you do not believe such calculations are adequate, contact the experts. For a start invite to my appraiser, he will be able to calculate you the amount that you will be able to rely upon the sale of the object. And then proceed all along the same formula: total amount divided by the number of meters.
Don't want some stranger came into your home at such a delicate issue - go to the professionals themselves. For this you need to contact any real estate Agency for advice. What are the source data of your property - the square footage, distance from public transport, floor, area, etc. Experts will make you an estimate on the total cost of the apartment, with whom you will continue to work independently.