Friday is member's day and Saturday is the main day.
I've found the price list for last year's Ponycon..
Last years (2011) prices are below:-
Spoiler
SATURDAY 22nd OCTOBER – MAIN DAY
* Adult Pass £10
- General admission
* Child Pass (4-12 years) £5
- (Children under 3 years – FREE)
* Stalls - £25
- Trade or Display (price includes admission for 1 adult on Saturday only) - Limited Number Available
FRIDAY 21st & SATURDAY 22nd OCTOBER – MEMBERS DAY & MAIN DAY
* Weekend Adult Pass £18
- Friday & Saturday Admission - Limited Number Available
* Weekend Child Pass £9
- Friday & Saturday Admission
*Weekend uplift ticket £5
- Available to stall holders only, allows entry on Friday 21st October