I cannot remember the cost, the best I can remember is that it was in the range of 700 US for five of us. We also drove ourselves, following Jason from Fantasea Dive. I do know though, whatever the cost was, it was money well spent and cannot wait to do it again this summer.
Children are not any less, as they are still taking space on the boat and, I believe, that there needs to be a ratio between the amount of people on the boat and the operators. In addition, some operators will not take children under the age of 12.