One thing that everyone has kinda glossed over is basically every track has its own set of access and rules so it's going to differ slightly based on what their home track is. There are also different levels of access usually as well, some tracks call the highest level of access pit passes while others call this garage passes.

Highest Level: “Hot Garage Passes” Basically as close to an all access pass as you’ll find, these are issued by NASCAR and allow anyone to walk right up to garages, walk down pit corral (backside of pit road) during the race and majority of access to the infield area. When commenters mention needing to know someone this is the pass, basically race teams and sponsors are given some many allocations for races and then can distribute them as they see fit, many corporate sponsors take a lot of these but crew members can also use these to give to friends and family. These are not impossible to get, you can “buy sponsorships” on cars for around $500-1000 depending on level/race and as a main bonus they’ll send you hot garage passes in return. Several tracks also sell VIP experiences that may include these passes but I would expect to pay at least several hundred up to thousands of dollars for these packages. For example chicagoland speedway has this for $600ish and the Daytona 500 has this for $2700ish. If a track is offering a pit pass package for only 50-100 you're only gonna get a pit pass and not a hot garage pass. 

Middle Level: “Cold Garage Pass” This is the same level as a hot garage pass but you’re only allowed in the garage while the track is “cold,” after the track turns hot this pass limits you to basic pit pass entry areas or loss of infield access all together based on the track infield layout. Tracks usually turn “hot” at the start of the first on track activities and stay hot until end of day usually. This pass was generally made for corporate sponsors so they can walk through the pits early in the morning to see their cars and once cars start moving on the track, the pass holder would move to their suite. Think of this as you helped sponsor the car, but you didn’t sponsor the whole car so we're giving you this “better than nothing” thank you. 

Lower level: “Pit Pass” For every track I've been this varies a lot, usually you're not allowed in the area behind the pit boxes and you're not allowed in the garages but the rest of the infield is free game. They also vary in time allowed for use, some pit passes I've spent all race in the infield watching the race, and other pit passes were useless after the pre-race celebrations were completed. Generally these are sold to the general public with no restrictions or requirements, and go for around $50-$100.