Mustad, You can probably fix your issue for a lot less than $500. What I would suggest is install the shim that helps the most. If I remember correctly Benelli shims fit between the receiver and the stock, just ahead of where your hand grips the stock, correct? Then you remove the buttpad, and install another tapered shim between the stock and the buttpad. You can make a tapered shim from different materials, but I would suggest either a sheet of plastic, or a "trim-to-fit" buttpad like you find in gun shops(or Cabela's, Bass Pro, Midway USA, or Brownells). If you go with a sheet of stiff plastic you can use a woodworking plane to adjust the taper. If you use a ready made buttpad you will need to find a way to trim its mounting plate (assuming it has a mounting plate, for this a wood plane might work well also). If it does not have a mounting plate trimming the rubber will be more difficult.
If none of these sounds workable for you, try calling Brownells and asking their advice. They have some very knowledgeable people who mayhave some great ideas. Good luck!