Levi's Stadium was publicly financed and is operated by Santa Clara County. PSL costs are or were likely restricted as part of that agreement with the team.
MetLife was privately financed, but also costs shared by two teams, Jets and Giants. Both teams sold PSL's, so rights to each seat in the stadium could effectively be sold twice, which obviously helps keep prices down for fans on each side.
PSL's have historically to this point not turned out to be good investments for fans, as the prices fall on the secondary market. Some teams, like the Jets for instance, have had some problems with fans simply walking away from (defaulting on) their PSL's, since they're financed for many years like a mortgage. They become like home owners that are under water. No longer can or want to make the payments and can't sell for enough to cover what they owe, so they just walk away.
