Data nodes plus EBS storage for a managed search cluster.
Search clusters get expensive when stale indices force more or bigger hot nodes. Roll old data to warm/cold tiers and size hot nodes to live queries. Related: log retention calculator.
Free OpenSearch / Elasticsearch cost calculator — data nodes, hourly price and storage to your monthly cluster bill.
Per data-node-hour for compute, plus EBS storage per GB-month attached to each node, plus dedicated master nodes on larger clusters. You pay for the running cluster continuously, not per query.
Use index lifecycle management to move or delete old indices, keep only the shards you query hot, and right-size node memory to your searchable data. UltraWarm and cold storage tiers hold rarely-searched data far cheaper than hot nodes.