You cannot Purchase Lightroom Mobile outright.We Recommend you to keep Creative Cloud Photography Plan to take advantage of Lightroom Mobile Sync with Desktop.
There are other apps like Photoshop Fix,Photoshop Mix,Voice,Portfolio,Slate that are bundled in the Package.
Lightroom mobile is only available with Lightroom subscription.
In case, you cease the subscription, you will not be able to use Lightroom mobile.
That used to be true but no longer. The most recent release (Lightroom mobile 2.0 iOS) in Oct 2015 changed all that.
You can now use the Lightroom Mobile app without a subscription but you'll need to subscribe to sync with Lightroom desktop.
"New in Lightroom for mobile for iOS, version 2.0:
- Lightroom for mobile for iOS can now be used on your phone or tablet without a paid membership, coming soon to Android."