What are the costs involved in testing a subgraph on the Solana network?

The cost is not 3000 GRT because you can withdraw this 3000 GRT. It is more like an optional deposit that you can make for better QoS. The cost is the transaction fee of deploying your subgraph as well as a 1% curation tax (if you choose to do so) which of 3000 GRT is 30 GRT. This is what will be burned. i.e. if you want to curate your subgraph with 3000 GRT, you will actually need 3030.