How I Cut My AI API Costs by 70% (With Real Invoice Numbers)

─── I've been building an AI-powered IELTS speaking practice app. The core pipeline is straightforward: Whisper for speech-to-text → GPT-4o for evaluation and feedback → TTS for audio response. Before launch, I ran the numbers and nearly killed the project on the spot. At 100 daily active users doing 3 sessions each, API costs alone would eat ¥3,000+/month (~$420). No revenue yet. Just burning cash. Then I found SubRouter. Here's what happened after a few months of real usage. The Actual Numbers No cherry-picking.