I would like to go from Ushuaia to Cape Horn by sailing boat. Would be a one week trip. Is there anyone who know how to arrange it in a more or less cheap way?
The best options you have is looking for sail boats requiring a crew.
You should be careful with that. Make sure the captain is experienced enough. Cape horn is not known by being easy. But you're aware of that probably.
They will, most likely, be looking for someone with experience as well.
Things can go in many ways. It's not easy to get a good crew sometimes so it's possible that they don't ask money. The most usual way, I would say, is food cost sharing.
There are several websites / forums where you can search:
There are also "professional" websites that list ads like:
Some of them require you to pay a fee to answer the ads
Another option is going to a near by marina. There are usually classified ads for several topics, namely crew seek. Depending on where you are it may be easy to find a boat going there. If you are in an area, with mainly local sailors you will hardly find someone going that way. If you're in Azores, with a lot of blue water sailors coming/going everywhere than your chances may increase quite a bit.
If you are in Ushuaia or have a quick way/cheap way of getting there, considering lot of boats depart to Cape Horn from this place, I would definitely visit the local marina/harbour. There is nothing like meeting a sailor face to face. Last minute opportunities are not uncommon.
I wish you good winds for your search.