We don’t have kids but we have a dog that we would never kennel so our circumstances for wanting one is different but I’d imagine a family of four would soon pay for itself
You would have to factor in the cost of using it (pitch fees etc.), running a caravan and using it on holiday isn’t free. Sites that have good amenities tend can be quite pricey, if you want a pitch were you don’t have to fetch your own water etc. (Your on holiday after all!) then it’s more on top of that.
Compared to going to the EU on a plane, it isn’t necessary cheaper, it’s just different and they offer flexibility (e.g. you can take more stuff, dogs etc.). You really have to use them before they come into their own.