For one of the lowest pilgrimages from the US, check with Fr. Joseph Galic whose email address is josephgalic @ aol.com
I'm uncertain of the cost as I've never traveled with him but he personally told me he is associated with a group in Chicago that tries to keep costs to a bare minimum.
Another possibility is to organize a group yourself. As Jenn mentioned, many pilgrimage companies will provide for one free guest given that you organize a group. Check with Catholic Journeys out of New Orleans (http://www.CatholicJourneys.com
). In the past, I believe they may have provided one free guest for every 8 pilgrims. It is common, but not necessary, for the free guest to be a priest to accompany the group.