A professionally sharpened lawnmower blade is actually sharper and better balanced usually than a new blade. When a blade is professionally sharpened it is first cleaned from all vegetation, meaning dried grass, with a wire wheel on a grinder, or other type of professional cleaning machine. After that operation it is then inspected for cracks and checked for straightness followed by a professional sharpening on a machine specifically designed for sharpening lawn mower blades, usually involving a grinding wheel or an abrasive belt type of sharpening machine. For the most part the abrasive belt type of machine removes material faster than a grinding wheel which the grinding wheel type of machine can often burn or blue as it is called the metal in the blade causing the blade to lose its temper or hardness, whereas the faster material removal of the belt sander causes less heat transfer to the blade, resulting in a cooler metal removal process and less chance of blueing the metal causing the blade to lose its temper which would result in the blade not staying sharp as long. Also a professionally sharpened blade will result in a perfectly machined bevel, the cutting edge, with the machine angle usually set at a 30 deg. angle, as well as all facets of a mulching blade sharpened as well. That angle can be difficult to achieve manually on a bench grinder, but a machine will hold that angle the full length of the cutting bevel.
This process of professionally sharpening a blade can run anywhere from 8 to 15.00 per blade depending on what area of the country and what steps are involved but a good price I have seen for this process is 8.00 to 10.00 for one blade 15..00 to 20.00 for 2 blades and 20.00 to 30.00 for 3 blades. Also a blade done in this manner is just as good if not better than a brand new blade since it is sharper and also individually balanced. Factory blades are not individually balanced so in my opinion it is cheaper to have your blades professionally cleaned, sharpened and balanced since a blade serviced in this way is much cheaper and just as good if not better than a brand new one.
some websites to see the process of professional sharpening and balancing a lawn mower blade are videos on You Tube, as well as BurrKing.com for the abrasive belt sharpener type of machine the Burr King X400 or Magnamatic.com the abrasive grinding wheel type of machine the Mag 8000 or 9000 as well as the Magnamatic blade balancer.