  The Android market is a place where developers offer paid and-or free apps to
users. Free apps are interesting to users because they can try them immediately
without incurring a monetary cost. However, free apps often have limited
features and-or contain ads when compared to their paid counterparts. Thus,
users may eventually need to pay to get additional features and-or remove ads.
While paid apps have clear market values, their ads-supported versions are not
entirely free because ads have an impact on performance.
  In this paper, first, we perform an exploratory study about ads-supported and
paid apps to understand their differences in terms of implementation and
development process. We analyze 40 Android apps and we observe that (i)
ads-supported apps are preferred by users although paid apps have a better
rating, (ii) developers do not usually offer a paid app without a corresponding
free version, (iii) ads-supported apps usually have more releases and are
released more often than their corresponding paid versions, (iv) there is no a
clear strategy about the way developers set prices of paid apps, (v) paid apps
do not usually include more functionalities than their corresponding
ads-supported versions, (vi) developers do not always remove ad networks in
paid versions of their ads-supported apps, and (vii) paid apps require less
permissions than ads-supported apps. Second, we carry out an experimental study
to compare the performance of ads-supported and paid apps and we propose four
equations to estimate the cost of ads-supported apps. We obtain that (i)
ads-supported apps use more resources than their corresponding paid versions
with statistically significant differences and (ii) paid apps could be
considered a most cost-effective choice for users because their cost can be
amortized in a short period of time, depending on their usage.
