I work for a coach hire company and the normal rate is around £400 - £500 for a day, it may come in slightly cheaper, but the problem is, that if it is used for a school trip, it really is a day hire, because it can't be used for anything else during that time.
The running costs i.e. maintenance, fuel, drivers salary (he is paid when he is waiting around at the venue), all mount up. 30 kids on a coach costing £400 is £13 a head, before you even start to pay for the venue, so £20 is probably about right.
We don't really take on many school trips, because unfortunately we can get a lot of damage and mess (think chewing gum on seats and seatback trays broken!). On one school trip the coach was set fire to - a kid in the toilets, smoking and putting the end in the waste bin! Where are the teachers? All sat at the front and not throughout the coach!
There are companies providing cheap coaches but I would be seriously concerned about any company that charges £100, they must be cost cutting somewhere to do it at that price and it is probably on maintenance. Some old vehicles don't have three point seatbelts, just lap belts, they also are not maintained very well, despite being 'checked', by the Local Authority and on their supplier list.
The coach company isn't spoiling your trips, they are trying to make a living too, just like everyone else!