If you only want Illustrator along with Photoshop then you will be paying for another subscription.
Manage your membership and payments | Creative Cloud
So it will be more profitable for me if I take a full pack subscription?
And will I lost my previous payment?
"profitable" is not the right word for this since that depends on how much you earn from your use of the purchase. If you mean getting the full subscription might be more economical then that is more likely the case when you get to having three applications. In your case you have Photoshop, which you could have gotten as part of the Photography plan which is half the price of getting Photoshop alone. That along with the Illustrator subscription as a single application plan would still be more economical than going with the full subscription.
You should review the different plans to gain a better understanding of what is offered and what you can arrange for your own needs...
Creative Cloud Plans
All Cloud Applications: http://www.adobe.com/creativecloud/catalog/desktop.html
If you want to change something with your membership then contact Adobe Support thru chat and they should be able to assist you and avoid extra costs.
Chat support - For the link below click the Still Need Help? option in the blue area at the bottom and choose the chat option...
Creative Cloud support (all Creative Cloud customer service issues)
http://helpx.adobe.com/x-productkb/global/service-ccm.html ( http://adobe.ly/19llvMN )