Hi Ed,
A good guess as to the cost to convert is between $3,000 and $3,500. I can't give you and exact cost because I had my nearly new front 315 put on the tags. I had a small crack in one of the front wheels so that meant one of the fronts had to be broken down and put on a tag wheel. Also, front sway bar bushings had to be replaced and tag shocks. With the extra work involved, it changes the labor cost. The costs for tire conversion parts are as follows:

Tires 2 @ 645
Wheels 2 @ 699
Studs 20 @ 8.59
Nuts 20 @ 4.69
Equal 2 @ 25.54

I also received a credit for the two old tag tires and the one good 315 wheel. As I previously mentioned, it is well worth the cost.