Yes the price is the same buying at the gate or ahead of time. Still it saves time to buy ahead and skip the ticket booths.
Because no websites actually disount tickets they tend to not deal with short visit tickets. They make money by avoiding sales tax and split the difference with the customer.
For single day tickets the are a few option but probably won't work for you. Does you company offer them to you at a discount?
Are you a teacher in California?
Are you in the US military?
Are you a student in a souther california university?
There are special offers for the above groups.