Per character, not per credit
ElevenLabs sells you credits that convert into characters at a rate shifting with plan, model, and whether you cloned the voice; on the lower paid tiers the effective rate works out to around $100 to $300 per million characters. SpeechifyAI is from $6 per million characters on Scale on flat per-character billing: the same number however many spaces or SSML tags are in the script, no model multiplier, no credit math, and no surprise when a long document costs five times what was budgeted.