Fair Value formula

I am trying this fair value formula, but I keep getting the following error,

An error occurred while trying to evaluate the price calculation formula. Check formula syntax.Are case statements not allowed in fair value formulas? Is there a way to rewrite this?

CASE

 WHEN {quantity} BETWEEN 1 AND 49 THEN {quantity} * {fairvalue}

 WHEN {quantity} BETWEEN 50 AND 99 THEN ({quantity} * {fairvalue}) * 0.6

 WHEN {quantity} BETWEEN 100 AND 199 THEN ({quantity} * {fairvalue}) * 0.47

 WHEN {quantity} BETWEEN 200 AND 499 THEN ({quantity} * {fairvalue}) * 0.37

 WHEN {quantity} BETWEEN 500 AND 999 THEN ({quantity} * {fairvalue}) * 0.27

 WHEN {quantity} BETWEEN 1000 AND 5999 THEN ({quantity} * {fairvalue}) * 0.23

 WHEN {quantity} BETWEEN 6000 AND 9999 THEN ({quantity} * {fairvalue}) * 0.13

 WHEN {quantity} BETWEEN 10000 AND 24999 THEN ({quantity} * {fairvalue}) * 0.1