If it is $400 for regular old tint, then it is over priced. For something high-end like Huper Optik, then the price sounds about right. How many windows are being tinted? Is the windshield beign tinted?
I paid $400 for Huper for all 5 windows, the windshield was an additional $200.
I doubt you will have any warranty problems. Make sure they dot-matrix shows through on the sides and that they go up to the top of the glass.
2011 E93 - Space Gray - 6MT/Silver Novillo/
OE Tuning/Gruppe M Intake/ACS Springs/Vorsteiner CF Type I Rear Diffuser/Textured Matte Black Custom Powdercoated OEM 359s/Painted Reflectors/Matte Grille, Hood Vents and Side Markers/Arqray Race Exhaust/MS Pulley