As far as for cost, take how much you paid for materials, equipment maintenance (for your sewing machine, maybe $5/hr) and labour ($15/hr or whatever you want to pay yourself). Calculate the time per polo, times 4, add all the costs up and then multiply by whatever profit margin you'd like (15%). Boom - a price!
I would be interested in trying them out! The things that would be of a concern for me is the quality of the polos (fabric, stitching), if they fit my horse and the rhinestones staying on (and in their settings) in the wash.
I have a clinic coming up (I don't know if you've heard but Robert Dover is coming to Calgary - it should be an excellent clinic! Auditor forms are on the ADA website) and I could test them for you I have contacts at a few high end tack shops in the area (Top Hat n' Tails and HorseLife) and I'm sure that if I like them they would be willing to take up the line and sell them in their stores (if you're interested). This is definitely something that the dressage crowd around here would like! Especially if they can withstand the wear and tear of everyday use!