This I have done on occasion with good result, but it takes a little time and effort to do. Use a clear epoxy resin. You can buy it at Lowes or other hardware stores. Use a thickner to make it into paste form. Some paint suppliers sell the thickner. You may be able to find a paste form of epoxy to use but color may be a problem. I color the epoxy by adding pigment to darken it to a brown nearing the color of the wheel. Clean the area and apply the epoxy. You will have to devise a method of holding the material in place like masking tape as epoxy sags. After the crack is filled, shaped and sanded. Use a brush or air brush to apply color to the wheel to match the patern of the wheel. Reclear coat the wheel with eurothane clear. Its an envolved process and to pay someone to do it would be more than buying a good one but if you have plenty of time and want to do it may save you some money.