Andy,
The demand for any one species is probably not large enough to economically raise them as 'gem' specimen shells.  The start up cost would be very high until one could get a consistent product, but the price would soon drop.  As food or pets or for conservation purposes the demand may be high enough to sustain production, or the costs must be subsidized.

Coins are graded fairly precisely with books showing pictures of each variety of American coin in each of many conditions.  And the price goes up on a very steep curve with each slightly better condition as the best is approached.  However, shells are more like ancient coins which were individually struck from non-standardized dies.  These use the same general grading, but it is much more subjective and the overall look of the coin may be a better price determination than the amount of wear. 

Allen Aigen
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