I like this activity. I can imagine using it with my 8th graders when we are covering linear functions. Rotating through stations and working in groups is great for that age-level student and it is a topic they can relate to-cell phones!
I would not really change too much about it. The last question in the analysis section, #6, I would consider using as an extension if needed. I think the main activity, with class discussion, would take a class period.
I noticed another reply mentions graphing calculators. My gut instinct would just be to do it on paper, but it is something I would consider having the students do if they were familiar enough with them.
Thank you for this great lesson which applies real life situations that will spur the student's interest. I liked your various formats of presenting the information with tables, graphs and equations. I added a couple more with the Now and Next, Recursive and patterns. I eliminated the stations for my class because my students work at different paces and will not all be working on it at the same time. I also included the options for calculators and computers using EXCEL. I like to have my students present their findings to others and have to defend their decisions. With this activity their is no definite right choice, they just have to defend the plan they choose. This activity can also be extended to the many other options that are out their and what they would like to see. This is good practice in making educated and informed decisions about choices they will need to make later in life.