Adobe Illustrator is a subscription design program, which means there isn't a one-time purchase option. You can get it at as low as $19.99/month with an annual plan.
Adobe Illustrator is part of Adobe's Creative Cloud, and you must have a subscription to use the program. The Adobe subscription is paid monthly or annually, and with this payment, you get the latest version of the program, fully updated. Any update that comes out does not require an additional payment.