Update some 10.000 fields depending on City field

Update some 10.000 fields depending on City field

I have to monthly update the shipping Cost to a certain City in Accounts

Need some kind of script / tool   / or third party addon      to do something like:

If City = "New York"    then Shippingcost =100
else
if City = "Houston"     then Shippingcost = 200
else

etc etc
with upto 250 cities..

PS The file contains plus 60 K records/accounts

Thanks for any ideas !
William