Hi @Lutter-Yin, Qi ,
You are correct, using Azure Synapse will require some amount of cloud services to be running in Azure. There's no way around this since it's a cloud based service.
Likely much of your cost was from keeping the lab environment running for an extended period of time.
The costs are in a large part probably from the VMs, which could cost over $300 per month (each) if you keep them running.
My approach to those labs is to prepare and read as much as possible before running the lab and then ensure clean up afterwards.
You can set cost alerts in Azure to email you when your costs go above a given threshold - I always use this feature.
If my company was serious about using Synapse I'd get buy-in from my manager and ask them to be prepared to pay for it. However if you're a student I appreciate that may be out of pocket.
(I appreciate 300 Euros is a lot, but compared to some cyber courses it's not bad!)