Depends what you perceive value as I guess.
We paid £40 each online for Zipworld (velocity.) There are two ziplines, but it takes an age to get up the mountain/hill to the top, its almost like a little tour. I mean, being a self confessed adrenaline junkie, I thought it was good value. No roller-coaster or drop tower could come close to this Zipline, Compared to skydiving, bungee jumping or base jumping, it's actually really good 'value.'
There is a lot to do around there too, You have bounce below, both of the zipworlds, plenty of climbing/hiking, plenty of decent hotels and pubs, now this alpine coaster... Sure I'll give it it go next time I'm up there. £15 or not.