For their fans, owning a piece of the team is meaningful.
Fans spend money on team stuff. What is a PSL worth? A plain t-shirt costs around $10, but people will spend $60 for an authentic, official NFL t-shirt with a Bills logo.
Frankly, if the NFL would allow publicly owned teams (other than the Pack, who's grandfathered in), and the Bills were up for sale, I'd spend $300 on a share. Considering how much I've spent on DirecTV and Sunday Ticket, that's pretty small potatoes.