Waste money? I imagine that in school workshops work out far, far cheaper than anything similar that parents might be able to book out of school time. For example, a few years ago we had a special day for the children that consisted of 2 workshops- one art based and one with visiting animals- plus other activities. Cost to the parents? £1 per child. The children loved it, they talked about the animals particularly for weeks after, and had an art project that they took home. I wouldn't call that a waste of time!
Unfortunately, consulting parents never really works. Have you ever tried to organise a get together of more than around 20 people? I have and it's a nightmare organising dates that suit, and venues, and arranging the activities. In a school there are more like 200 people to please- some of whom think any non curriculum activity is a waste of time, others want their child to do more sport, yet others want art/music/drama.
Not to mention, when our school at least have consulted parents (eg by offering them 2 choices and going ahead with the most popular) you run into problems because the ones who didn't vote for that one feel disgruntled and therefore don't want to pay!
Ultimately, the school must do what they think is in the best interests of the children as a group,and if they think an activity will benefit them, go ahead and book it. Of course, with a take up as low as 10%, the activity should be cancelled and a note made that either the cost was too high, or the activity not appealing enough.