Ok, I need help with putting values of checkboxes in the code,

Heres an example of how my form wil l look like:

Ok I wanted it so The Trivium checkbox would obv equal to £50, so if I checked it, it would be selected and the price would be displayed in total price, and for the part where it says number of adults I wanted to set it so each adults would cost £5+ticket, as an example, below it shud say number of child but I forgot to rename, anyway I really am stuck with the checkbox and how to put those values in.

It's a bit messy there so il try and explain better,

I guess for the first part I would like to find out how to put values in checkboxes and calculate that price, example would be

Trivium select > Current Price in total is £50 (£50 per adult), for 2 adults price would be £100, I guess the number of adults should be default 1,

Price per Child is £25

So If they selected 2 adults and 2 children the total price would be £150, if my maths is correct :)

Anyway my design is a lot bigger than that but if I can figure out how to do those few things then I should be able to do it by myself

Thanks anyway