Physical damage will not void the warranty.
Different stores charge you with different amount.The cost to fix a broken screen of a iphone is usually between 50to 100 for parts and labor charges.
Most cellphone repair shops can do this front panel replacement,
but make sure they put genuine Blackberry,
or Nokia parts in it so that,
if you`re still under warranty,
you will not void it.
The better option is to approach the dealer rather than an ordinary store dealer who may not fix the original parts which later may cause trouble for you