This is Raspberry Pi 4 territory. Your monthly cost is the price of hardware, per month. If you are on a home 1G connection, your home bandwidth is as good or better than what you are getting on DO. At the end of a year, you would have a 12 node cluster.
And home connections are all asymmetric, you may get a gig down but I've not seen anything outside business class with over 50 up.
My 1G connection sees 250M up and down continuously.
https://www.jeffgeerling.com/blog/2021/pine64-and-radxas-new...
https://www.hardkernel.com/shop/odroid-m1-with-8gbyte-ram/
PS Shame on you Radxa for naming your module CM3, shame. There will be a flood of RK3588 based devices on the market in the next 6-12 months. A low end PC would replace 10s of these devices.
My point was that a 4 vcpu instance is overpriced and also a very small amount of compute. Most cloud costs are 1/3 to 1 of price of hardware, per month.