In The Network if you are paying for a query you decide the price you are willing to pay. Plain and simple. I’d take the amount you are willing to pay per month, divide it by your expected query volume, and set that as your maximum budget per query. (This already is or will be available as configuration in the Gateway)
You do get to decide your price, but there is no guarantee that any Indexer will be willing to serve queries at that price. That side of the equation is up to them. If the Gateway can match you to an Indexer willing to serve a query at or lower then the price you are willing to pay, you will pay the lesser of your budget and their price. You will always pay some amount less than the budgeted price. Possibly significantly.
As a consequence of the above, a lower price makes the pool of Indexers available to you smaller, which may affect the quality of service you receive. Seeing high query fees may attract curation and big name Indexers to your subgraph. A low price may leave you with only budget Indexers to choose from.
It’s a dynamic market, and it’s in your control. There is no maximum or minimum amount specified in the protocol or in our Gateway. (There was a price specified in The Gateway before, as stated by @Josh-StreamingFast, but this was just a price we paid for test queries and wasn’t meant to be norm-setting).