You don't need to be a Costco member in order to get the discount. You can order an adult ticket online:
Adult:
http://www.costco.com/Browse/Product.aspx?Prodid=11206458&whse=BC&Ne=5000001+4000000&eCat=BC|21124&N=4012511%204294912640&Mo=2&No=1&Nr=P_CatalogName:BC&Ns=P_Price|1||P_SignDesc1&lang=en-US&Sp=C&topnav=
Child:
http://www.costco.com/Browse/Product.aspx?Prodid=11206460&whse=BC&Ne=4000000&eCat=BC|21124&N=4012511&Mo=35&No=22&ViewAll=36&Nr=P_CatalogName:BC&cat=29676&Ns=P_Price|1||P_SignDesc1&lang=en-US&Sp=C&topnav=
Also- Costco sells a 3 day pass good for 55 attractions called the GoSanDiego Card. Legoland is included:
http://www.costco.com/Browse/Productgroup.aspx?Prodid=11219002&whse=BC&Ne=4000000&eCat=BC|21124&N=4012511&Mo=35&No=34&ViewAll=36&Nr=P_CatalogName:BC&cat=29676&Ns=P_Price|1||P_SignDesc1&lang=en-US&Sp=C&topnav=
Adult:
http://www.costco.com/Browse/Product.aspx?Prodid=11206458&whse=BC&Ne=5000001+4000000&eCat=BC|21124&N=4012511%204294912640&Mo=2&No=1&Nr=P_CatalogName:BC&Ns=P_Price|1||P_SignDesc1&lang=en-US&Sp=C&topnav=
Child:
http://www.costco.com/Browse/Product.aspx?Prodid=11206460&whse=BC&Ne=4000000&eCat=BC|21124&N=4012511&Mo=35&No=22&ViewAll=36&Nr=P_CatalogName:BC&cat=29676&Ns=P_Price|1||P_SignDesc1&lang=en-US&Sp=C&topnav=
Also- Costco sells a 3 day pass good for 55 attractions called the GoSanDiego Card. Legoland is included:
http://www.costco.com/Browse/Productgroup.aspx?Prodid=11219002&whse=BC&Ne=4000000&eCat=BC|21124&N=4012511&Mo=35&No=34&ViewAll=36&Nr=P_CatalogName:BC&cat=29676&Ns=P_Price|1||P_SignDesc1&lang=en-US&Sp=C&topnav=