I'm not debating how much it costs to make, because engineering it in a way to make it work while allowing the hook on the receiver to remain is probably no easy feat.
However, you have to consider several factors here:
1) If you wanted to use a hopper on a Tippmann, you can probably find a 98C for $70 or less and have a whole extra gun as a backup.
2) While I have no particular love for the Cyclone (I converted all of my A5's to RipClips shortly after the first time I used one), for most people's purposes it works just fine. And, with a Rip Clip adapter available for $30-40, (less if you make your own), it seems like that direction makes more sense since it already uses A5-style hoppers.
3) There are so many other affordable center-feed entry-level guns now that dumping $70 in to an A5 to make it closer to a center-feed is borderline idiocy.
So really, the only way people I can see this selling to are MAYBE the people with Phenom's who would rather have a hopper, which isn't going to be many, since the reason most people go with the "tactical" looking guns in the first place is not to have a tall hopper setup on their gun.
So I understand why it's $70: it's hard to make and will only appeal to a niche market (nothing we're not used to here on MCB). But I question the wisdom of even bothering because I'm not sure that niche market would be willing to pony up $70 just to keep the hook on their receiver when there are other options like the RipClip or the other hopper adapter.