Between about $25 and $40 depending on how you configure the machine. GCP doesn't offer exactly the same configurations as DO.

Despite me working on GCE, I have to give a shout out to all the hosts like DO that include a hefty amount of network egress in the price. The network is heavily overcommitted (how many of you actually send X GiB/month/droplet?), less reliable, etc. but for those that use it to serve images for "don't care" it can't (yet) be beat.

Disclosure: I work on Compute Engine and Cloud and want your business.

This is a good tool for cost estimation - https://cloud.google.com/products/calculator/

Google Compute Engine instances will always be more expensive than DO/Linode because GCP offers so much more. GCE instances will only be cheaper than DO droplets if you will shut down the instances when you don't need them as you don't get charged for them when they are off unlike DO.

I'm in the same boat... prepping a handful of .Net apps to run under docker (dokku) and migrating from Azure host to DO... though I may well continue to use Azure's storage services, and Azure SQL... the pricing is a bit better than the alternatives.