To clear up your question regarding tint laws, the tint grade determines lawfulness as far as driver's side windows, not the tint itself.
You may have to go with a lighter than desired UV protection if you want all windows to be the same tint.
For most states with that law, I believe 20% is the max allowed for the driver's side windows. This was enacted back in the 80's for the protection of law enforcement individuals that had to approach a vehicle with tinted windows, and the occupants (or their contraband) could be concealed.
I've never been quoted any differently for the job except for vehicles with additional windows (minivans, SUV, etc.). If the shop does quality work, the price should not change. Find another shop.
Quality of tint is important. Cheaper film will discolor, and some even "lift" and "bubble" near the electric defroster elements on the rear window (many of you have probably seen this).
Regardless, ALL tint will scratch, so greater care must be taken to protect the film. Ammonia-based products such as Windex cannot be used when cleaning or discoloration will result.
I am considering tinting the windows on my 300E, as Texas summers (and lack of covered parking at work) makes the car unbearable to enter in the afternoons.
Be prepared for the added care required to keep your tinted windows free from damage.
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car