You can't change just the CPU - it's permanently soldered so you must swap out the entire mainboard. It looks like the boards with i7s run between $225-300 depending on speed and whether or not you want discrete video or integrated.
If you now have a 65W PS, you'll need 90W or 130W for an i7 board depending on integrated or discrete video, so add another $30-50 for a power supply.
If you're moving from integrated to discrete video, the heatsink assembly will need to be replaced - add another $30 for that.
If you can't swap the parts yourself, add $100-150 for labor - so all in somewhere between $400-500 to upgrade.