Shell Programming and Scripting

i am able to achieve using if and elif. but we have 2000 hard code values then the code is big
based on the forums, tried sed [sed -e 's/10/6000/' -e 's/20/2000/' and so on] but that is also code is big.
Any direct command which give the  ouput.
sample example

a=we get some value

echo $a

if  $a value is 10 then 6000
elif  $a value is 20 then 2000
elif  $a value is 30 then 2000
-
-
-
-
elif $a value is 20000 then 500
so on

ex: a=20
output =2000

ex2 a=30
output=2000