I would presume that between unique AN proprietary (trade secret and/or patented) chemistries and from having put some R&D effort in that the stuff can't be copied. Other products will just be different, even if making similar claims. There are probably many other substances and ways of inducing panic (urge to make resin to protect seeds) in plants, so there could well be other products that work as well or even better and/or are cheaper. But if there were good and cheaper alternatives available, I presume we'd all know about them, if not be using them.
That's a problem with many AN products - they just seem to be the best out there.
I wish for me a cheaper alternative to AN existed, especially bfx. Due to the cost i often do not use on some girls, but i always notice the difference in resin production and yield, although this is a small diference yield wise.
I am currently looking into changing my feeding schedual slighly, possibly following tangs easy feeding routine, so cheaper alternatives always get my attention.
Big respect always goes to the brave and talented growers who experiment with new nutrients and supplements. Good luck.