I've been to Alti a few times and agree that £15 might be a bit too steep, but you've got to understand that the non-stop coverage of games on the TV, and access online to virtually any other game is killing smaller clubs. Neutrals and occasional fans are going less, so in lots of cases clubs rely on a core of regulars over walk-up support. If Altrincham, for example, dropped prices to £10 and were getting a thousand paying, that's a third of the gate money gone, they'd have to increase attendance by 50% to break even.
I think you can make arguments both ways as to whether £15 for a non-league game is too expensive or too cheap based on a number of factors. It's pointless comparing it to what a Premier League seat costs as part of a season ticket, though, you've got to compare like-for-like, to the cost of an on-the-day ticket bought without a membership. On average they'd be between 2.5 and three times more expensive, so in that sense it seems about right, but in some cases the football will be three times as good.
Quite a few non-league clubs offer reduced entry, sometimes even a fiver, if you have a PL season ticket, but i don't know if i agree with this.