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I loved Cartagena and enjoyed the week we spent there though we divided our time between the old city and Getsumani.
We also spent time out at the Rosarios where I did some diving. I found it as expressed by the OP however, it was also ridiculously expensive.
The cost did seem high. I paid in pesos and with a credit card. The use of a credit card was more than the cash price. I get an instant notice when my credit card is used internationally and it listed the cost in dollars as $132 for the two tank dive trip. I would normally pay around $80 for a two tank trip in Florida but that is with my own gear. The Diving Planet price included the gear, dive insurance, a basic lunch, and travel to Rosario Islands. I figured gear rental to be around $25 in Florida. I already have dive insurance so that was no benefit to me. When I looked into the cost of a group day trip to Rosario Islands with the same basic lunch, the cost was between $90-100 on Viatour. Since I was planning the day trip already, I calculated that the actual value of the trip to me would have been $195 (80 + 25 + 90). This made me feel better about paying $132 for the two tank dive trip.