The number (general development, not just mobile) at my previous job was $40K/sprint. So, to me $100K sounds light for a company of any size.
You gotta take into account the fact that the general cost of development and the cost of developing a mobile app is different. I assume your number includes salaries, where if you build it yourself, is not a problem. In fact, I could probably go make a simple decent game in and only have to worry about the costs of publishing it (Apple Developer Program and/or Google Play's fee). But if I want more developers, then that cost increases.
My point is that it's not necessarily that expensive to build an app, it just depends on the type, the size of your team, and a few other things.
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