Upon which price should zakaat be calculated?

I am in possession of a collector's coin set of several silver coins that I need to include in my Zakaat calculation. I understand that the value of the coin set is dependent on the current market value of the particular coin set, and not on the market value of silver. Searching online, authentic retailers are selling this coin set at several prices, with the lowest price being R3 250.00 and the highest at R3 600.00. There are several different prices in between. I have emailed other popular retailers whose prices are not mentioned online, requesting their prices for this particular coin set. Insha-Allah, they should respond soon. Depending on the response of the other retailers and the current pricing information, how should I calculate the 'market value' of this coin set for my Zakaat calculation?