Short answer: the cheapest speech-to-text API for an EU startup is the candidate that passes your invoice-field accuracy gate at the lowest normalized cost for your actual audio. Don't choose from a public per-minute headline alone. Put every provider behind one small TypeScript interface, replay the same supplier recordings, reject transcripts that fail schema checks, and compare the cost of accepted results.
For an edtech SaaS, this is a weekly-shipping decision, not a research project. The concrete job here is awkward but real: a school administrator reads fields from a supplier invoice into a voice note, and the application turns that audio into structured invoice data. A cheap transcript that changes a VAT identifier, currency, date, or total creates review work. That failed result has no useful price per minute.
I would time-box the first pass to an afternoon. OpenAI, Deepgram, AssemblyAI, and Google Cloud can all enter the candidate set named in the question, but their names don't determine the winner. A current quote, the exact billing unit, EU processing requirements, and a fixed acceptance corpus do. I'm not sure which one will win for your microphones and supplier vocabulary until those inputs are measured; anyone certain without them is guessing.
