A tiered rate system fixes this entirely IMO.  Las Vegas does this and it seems quite effective:
<a href="http:&#x2F;&#x2F;www.lvvwd.com&#x2F;custserv&#x2F;billing_rates_thresholds.html" rel="nofollow">http:&#x2F;&#x2F;www.lvvwd.com&#x2F;custserv&#x2F;billing_rates_thresholds.html</a>