Well, assuming it might take around an hour to do, and assuming someone was willing to work on it for ~$10/hour (most skilled artists would charge more than that)... then around $10, which equals ~12,000 tapas coins. Even then there is never a guarantee anyone will want to do it.
If you make something like this into a contest then you would have to offer even more to compensate for the risk that an artist spends time working on it and doesn't win anything. It's more economical to just hire one person rather than making a competition.
Also it's more economical to just use normal currency (like via PayPal) rather than tapas coins for payment, since Tapas itself makes a certain profit margin on the coins. For example, it might cost you $10 to buy 'x' number of coins from Tapas, but when someone tries to withdraw them, they are only redeemable for less than $10 (maybe ~$8 or $9).