The cost of a day ticket for the Festival of Light on Sunday 9th December is £34.15 for one adult. For a child on the same day it is £25.60.
If you also want to go on the Santa Express (which you can't on that date as it's sold out, but in theory) that is an additional £5 for an adult and £14.95 for a child.
So for a family of 2 adults and 2 children, including going on the Santa Express, the day will cost you (without food) £159.40.
The equivalent day at Chester will cost £20 per adult, and £16.36 per child - total of £72.72 (plus the cost of the monorail if you want to go on that - unlimited rides for £4 per adult and £3 per child).
Not that people who go to Longleat are going to go to Chester instead. I thought it would be an interesting comparison considering Chester isn't particularly cheap itself, but is for me a greatly superior zoo and day out. Some people disagree, which is fine.