If you're looking for a cost-effective way to create speech for your chatbot, neets.ai is a very competitive TTS option. It's available in multiple languages and supports multiple formats including WAV, MP3, Ogg and FLAC. It's got a free tier with 25k TTS characters per month and a Pro tier with 100k TTS characters per month, so it can be used in a variety of situations. At $0.001 per 1,000 characters, it's a good option for high-quality speech synthesis.
Another great option is Unreal Speech, which offers high-quality speech at a lower price. It can export in standard formats like MP3 and PCM µ-law, and has a free plan that covers 625M characters per month. The API also lets you control speech speed and pitch, and it's good for a variety of use cases, from content consumption to customer service.