I see it all the time, particularly in advertising - "Chairs for under $100 each", "Return tickets to Melbourne for under $150", "Don't pay over $1000". It makes me cringe.
The words 'under' and 'over' should only be used to describe something that is physically 'below' or 'above'. For example, "The bear went over the mountain" and "I'd like to be under the sea".
When describing a cost, amount or figure, 'more than' and 'less than' should always be used - "Chairs for less than $100 each, "Return tickets to Melbourne for less than $150", "Don't pay more than $1000".
Try to remember this rule next time you book a newspaper or TV ad. You'll be making my leisure time so much more relaxing!