At a previous company, we literally did that, and also had to get our VM limit increased.
But it only gets you so far, its a cost optimization at the low end, but eventually it isn't "worth it".
You are checking your bandwidth use against the pool limit, then spinning up a new VM when you get close to the limit, but then back down the next month so you aren't paying for unneeded bandwidth, paying the extra $5 and not worrying about some custom price hack is a lot easier and less stressful.
1000GB Data consumption is 10$ (1.000 GB @ $0.01 / GB) but adding a Droplet for 5$ adds 1000GB to you Datapool.
So with adding cheap Droplets you could lower the Bandwith Cost 50%?