I buy my stamps at our resort gift shop. Last year they were about 95c for an international postcard, up from the previous year, so I would say about $1. They have the international stamps and will ask where you are sending it so you will get the one per postcard - no need to stack them to reach the required value for postage.
Wayne gave me the best advice ever about postcard. I mentioned how mine take about 2 or 3 weeks to get here and he recommended putting them into an envelope (same postage price) as postcards are treated as junk mail and non priority. Put it in an envelope and it gets here in a few days. I have to give credit to our moderator for this one.