It's a flat fee of $100/month, correct? What would be the best option if the amount of training you do is rather "occasional" (but simply using colab doesn't cut it anymore)?
I don't have specific experience with ML, but AWS spot pricing is by far the best deal last time i checked for GPU. You can get something much more powerful than a gtx1080 and get your task done more quickly. The downside is that at any time your instance can be shut down after a short warning signal to backup your progress, so it may or may not be suitable for what you're doing.
Does the price actually depend on whether you are using a GPU or simply an instance you choose? Let's say you need to do some work that will require a GPU, so you spend 5 hours setting up an environment, doing some light programming/experiments in an Jupyter notebook, downloading datasets, looking at the data. Then you train for an hour then one more hour looking at the data, drinking coffee, stuff like that. Then train again.
So you were using the environment for 10 hour, but only 3 of them in total were using GPU. Will you pay for 10 hours of GPU usage, or will only 3h be expensive and 7h cheap?
If you use a GPU instance you pay the cost for it whether not you use the actual GPU. If the GPU time is short relative to the other stuff you are doing (like data cleanup) it might make sense to do your non-GPU related setup on a different instance first.