There was a recent thread on this...
Lots of good info there, i2f said he went through for under $1100 total, more specific details in the thread.
I think the canel was built because when you are paying ships by the hour, paying for cargo ships fuel, taking a loss for every ship lost, it is much cheaper / safer to use the canal.
Taking the horn on a sailing boat might be cheaper, depending on how much ware and tear and damage you take around the horn. Other then that, if you can sail the whole way it is free. I would not consider doing the horn based solely on the cost. The horn is a major undertaking and by many accounts the most dangerous place to sail in the world. The number of private sail boats who have made the trip can still be easily counted I believe.
In my opinion, do the horn if you want to do it, and have the boat and gear for it. Otherwise, find the money and take the canal. Just my $.02 cents worth, but I have done neither yet.