a retreat is strongly recommended as part of Confirmation preparation, preferrable within a few weeks before the sacrament is celebrated. If properly planned and if the candidates are prepared for it carefully, and it is led by experience directors, it can be life-changing, and one of the most significant spiritual experiences for the candidate.
There is no way it has to cost money and involve an overnight stay, but ideally it should be at a location conducive to meditative relaxation, away from the normal place they have their classes or meetings.
If there is not adequate adult supervision (parents, not college kids) say one adult for each 10 kids of the same sex, don’t let your kid go near it. Know everything there is to know about the place, environment, those leading the retreat, and common sense security issues.
A word on expense: our kids had no trouble getting parents to cough up several hundred to a thousand dollars for travel involved with band competions, sports playoffs, livestock shows, rodeos and various other non-academic endeavors, but balk at a 30-50 dollar price tag for an lock-in close to home, to cover food and facility rental. Cannot get any parents as chaperones, so there will not be an overnight, although it is the one thing the kids beg for every year.
$120 is extremely reasonable for a weekend retreat, you are very fortunate. Price range for an average retreat center is $50 to 150 a night, depending on accomodations and food. If I could have paid somebody that much to take my kids for a weekend to a safe place 10-15 years ago I would have jumped at it.
If you do go along as a chaperone, stay with another group, give your own kid plenty of space, don’t hover, don’t participate directly in what activities he is doing. ideally, he should forget you are even there. resist the urge to force your kid into a discussion of what went on at the retreat until a few days, so he has a chance to process it, and wait until he is ready to talk about it.