The psychology of sales.
Less expensive items look like better deals when they are percentage (%) sales, whereas dollar amount ($) sales are the way to go for more expensive items. The rule of thumb is, if it's greater than $100, go with a dollar amount - if it's less than $100, use a percentage.
Example: if a shirt that costs $25 is goes on sale, it's better to use 20% OFF rather than $5 OFF. Or, if a computer costs $400, it's better to have a sale price of $80 OFF, rather than 20% OFF. Consumer psychology - use it with respect and your consumers will return the favor.