Section 327 has the best possible view on the upper deck and are easily the most coveted seats up there so they'll come with a little bit of a heavier price tag. (Roughly about $30 per ticket for most Tigers games).
I always try to sit here and I'm usually successful, you can often times buy the cheapest ticket you can get your hands on, down on the street and then sit behind home plate on the top level. The ushers aren't very thorough at checking tickets, I think they just want fans in the building.

The easiest and most efficient way to get to these seats is to enter thru Gate A, which I call the main entrance and then turn right down the first base side until you see the escalator and take it to the top level. Just follow all of the signage,it's not too hard to find your seats at this ballpark.
There are also plenty of stair cases made available for fans throughout the building and it's not too much of a workout.

P.S. - You can bring your own food and bottled water into Comerica as long as it's sealed correctly, they're not to thorough at checking purses for food items, either so get the wife or girlfriend to sneak you in something good. Also, check out Cheli's Chili Bar across the street from the ballpark on game day; they carry a huge crowd!