Three costs, not one
People price the initial index and forget the other two. Re-indexing on a schedule multiplies that build cost by however many times a year you run it, and query embedding is a permanent per-request charge that scales with traffic rather than with corpus size.
At high query volume the third line overtakes the first two, and the fix is caching rather than a cheaper model: in most products a small set of questions accounts for most of the traffic.
Keep it in proportion
Embedding is almost always the cheap part of a RAG system. The generation call that consumes the retrieved chunks typically costs an order of magnitude more, because generation models are priced far above embedding models.
Use this to sanity-check the index, then spend your attention on the retrieval budget, which is where the money actually is.