a Benedictine Cross is a crucifix, and embedded in the center is a two-sided medal with an image of St. Benedict on one side, and a cross with an acronym on the other side. the wording on the medal includes the motto of the Benedictines, and a prayer of exorcism (for deliverance from evil and the influence of Satan). the medal was struck by a previous pope as a jubilee medal. there is a complete explanation of the medal and its significance in the package when it is sold.
You cannot purchase a blessing, that is forbidden and is considered simony to attempt it. Once you purchase this or any sacramental you request a blessing on it from a priest to signify that it is used as an aid for devotion and spiritual progress focused on Christ. The article has no power by itself, but the blessing does because it comes from Christ through those enjoying apostolic succession. The particular blessing associated with this medal was permitted by that pope for Benedictine priests, and includes and exorcism which is a very powerful prayer of the Church. That remains with you as long as the medal is on your person or near you - purse, car, etc.
while you cannot purchase a blessing, you can make a donation to a religious order, and receive as a thank you give a sacramental such as a rosary or medal which has been blessed. the monetary value of the item is far less than the donation, so you are not paying for a blessing. also, as with a monastery near us, the sisters purchase the items in bulk, then ask the monks to bless them, then later make them available for others, who reimburse the sisters for the initial cost of the item. again, you are not technically paying for a blessing, but for the object itself.
the reason the exorcism is associated with St Benedict and his medal is that several times evil monks who objected to his enforcement of their rule and its moral norms, attempted to kill him by poison, which was thwarted by miraculous intervention.
