A useful range, with limits
A quick budget estimate for 1,000 words of plain English is about 1,300–1,700 tokens. Treat it as a range, not a conversion constant. Text with many numbers, symbols, URLs or code can have more tokens per word. Short common words and familiar phrasing can use fewer.
Why a word count misses billable text
Your API input includes more than the visible user prompt. System instructions, conversation history, retrieved passages, tool definitions and formatting may all be sent and billed. The model's output adds another count with a separate rate. A document of 1,000 words is therefore not the whole request.
Measure before a large run
Use the target provider's token counting method or inspect a small batch of actual API responses. Record a low, median and high case for your content. Price each with Tokens to USD, then multiply by expected volume. This catches expensive outliers that a single rule of thumb misses.
Worked planning example
If you budget 1,500 input tokens for a 1,000-word draft and 500 output tokens, illustrative rates of $3 input and $15 output per million yield (1,500 × 3 + 500 × 15) / 1,000,000 = $0.012. Use measured counts and the exact model's current price for a real estimate.