Let's talk about the Samsung Gear 360 (not the 2017 model SM-R210NZW with the reduced photo resolution). It's available for 100€ or less in Germany (as of Spring 2017).
The SM-C200 takes fully spherical pictures. It has a nice high photo resolution of 30 megapixels. It has (partial) support for the Google Open Spherical Camera API (in "Streetview" mode).
What are the issues that prevent it from being supported by the Mapillary app?
Also, when not using the app, what's the best way to
* take lots of pictures at a configurable interval?
* stitch them?
* fix the horizon if the camera is held at an angle?
* get GPS information into the pics (the OSC API provides this function)?
* do bulk uploads taken by this camera to Mapillary?
Stitching a large amount of pictures using their PC software (Cyberlink Action Director) is not very pleasant or efficient.