I was there with family in October, the cost at the gate is 28 Euros for adults, 18 Euros for kids, but if they are under 3 years they are free. There is free Siam Park buses that operate to take you to the park from various locations, check out the Siam Park website you can get more info on there I think. We just got a taxi each time we went, and then there is a taxi rank outside the main gate for home time. The park opens at 10am, we made it there about 10.15 each time and had to queue for about 15mins before getting in. If you are paying by credit card remember and take your passport !!
They close at 6pm but start closing the slides about 5.30pm, one at a time. The cans of juice are very expensive 2.40 euros a can !! So take some with you!!
The place is totally amazing, cannot wait to get back there next year.