Hi and welcome @vrck.
Now is too early to speak about prices, because Migration didn’t finish yet. All subgraphs working with hosted service now with 0 fees. Only several subgraphs testing decentralized network and as I know so far, didn’t make queries yet.
You can’t select a specific indexer as a user (as I know), this work does automatically by protocol.
So, accordingly with #1, the protocol will help you to match your ask to Indexers terms automatically. For now, you can find the cost models, but without any UI or useful tooling, after migration will end, I believe we will see some of them.
It depends on the specific Indexer’s cost model. Someone could charge fix, someone could charge accordingly with complexity and other parameters. It will be a free market, where you will be served by Indexer accordingly with your requirements to price, reputation (not on the first stage. but still), and so on.