You can purchase the monthly subscription of photoshop from : https://creative.adobe.com/plans
Price of photoshop or any indiviual subscription is 19.99$/month.
you can purchase complete Creative Cloud Membership in 49.99$/month.
If you already have license of CS3 or later version then you can get the complete Creative Cloud Membership in 29.99$.month for first year.
you can purchase CCM subscription on your Adobe id and after purchasing subscription you can signin to license the software.