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My front Bumper conversion cost me around $700 on my 2010, but I did the California special.
I found a matching GT bumper on Ebay for $200 + $85 Shipping
CS/GT Grill Surround for $130 + $50 paint
CS Lower Valance with Fog lights $290
Before

After

 

[Siber Express]

You will need this with a GT Grille Mustang Grille Surround GT 2010-2012 | CJ Pony Parts
Or This with either a CS/Grill or V6 Grille 2010-2011-2012 Ford Mustang GT Grille Adapter

 

[Siber Express]

Okay, and if I get the bumper and grille surround, do I still need a grille?

If you get the CS/GT Surround you can use the V6 grille, that is the AA Part number, with the BA part number you will be able to use any GT grille, and like was said before the Saleen Grille might not need either.