How to convert video format on cloud based application

I'm building a smartphone application which supposed also to take a short video and send it to a cloud-based-storage like the Amazon S3. I also have servers on the cloud (i.e. on EC2).

Let's say I resize the video file on the app (like WhatsApp does). I want it to be delivered to other users, with other devices (both cellular and desktops), so I guess I need to convert the video to other formats.

Using the Amazon Elastic Transcoder seems very expensive (about $0.015 per video minute) cause I plan to have a lot of video files stored on it.

What should I do? Use a format-conversion program on my servers (which also on the cloud)?

I don't want to stream the videos, but to give the files like in whatsApp.