They could raise prices in general, raise the minimum plan or have a pay-per-use ($0.2) like you suggested. I doubt there can be a acceptable price hike that can offset the decrease in usage volume.
Ultimately, these are stop gap measures and doesn't stop their decreasing ARPU from losing dollars to 3rd party services not run by them - text, overseas calls, apps, and media (songs, movies, ringtones), even voice calls and location-based services. Carriers have been projecting an increase contribution of ARPU from data services and text is a major component of it.
Anyway, raising prices to beat a downward trend is dangerous. Could make it worse.
