Question: Is there a TTS API that offers flexible pricing tiers and supports infinitely scalable voice generation for large projects?

If you need flexible pricing tiers and the ability to scale voice generation, WellSaid Labs is a good option. The service has a range of plans, including a free tier and options for Maker, Creative, Team and Enterprise customers. It can handle high-quality, natural-sounding audio and custom voice avatars, and is geared for content creators, marketers and businesses.

Narration Box also offers flexible pricing tiers, including a free tier with significant abilities. It can handle more than 140 languages and accents, with abilities like context awareness, emotive styles and fine-grained voice control. It offers both basic and enterprise-level plans, so it's good for everything from e-learning to commercial use.

Neets.ai is another contender, with competitive pricing models that scale. It can handle multiple languages and formats, and its models are tuned for low latency and high-quality speech at a reasonable cost. The service is good for international projects, and it offers REST and Streaming APIs for integration.