How did they come up with the figure of $0.40 per minute? I can understand why they say "per Minute" since you think in seconds (which would be too short), min

How did they come up with the figure of $0.40 per minute? I can understand why they say "per Minute" since you think in seconds (which would be too short), minutes (just about right) and hours (which would be too long). But that $0.40 charge per minute seems odd. Why not $1, why not $0.50, $0.25 or $0.10?