Earlier quoted context omitted.
You don't need to make an application closed source to have in app purchases. Look at mobaxterm [ http://mobaxterm.mobatek.net/license.html ] It is licensed under GPL version 3. It has a free version and a professional version which cost $69 per user. This does not break the GPL. You may already know this but it seems like a lot of developers think open source mean Free as in cost. [ http://www.gnu.org/philosophy/sel…
Although the GPL allows authors to charge for software, it disposes of all legal copyright protections that make such charges practical. Charging for GPL software doesn't mean anyone will pay you, it just means that everyone will get the free builds from someone else, which is even less desirable than releasing free builds since you lose control of distribution. Look at Red Hat and CentOS for case in point.
My favorite IDE RStudio is AGPL which offers a Open Source license that also allows them to charge for added features.
http://www.rstudio.com/pricing/
AGPL - The GNU Affero General Public License is a free, copyleft license for software and other kinds of works, specifically designed to ensure cooperation with the community in the case of network server software. [http://opensource.org/licenses/AGPL-3.0]