I have not raised guppies or other fish to feed to chickens.
About feed to guppy-weight conversion, a bit of googling found this paper for me:
https://www.researchgate.net/public...sion_in_Guppy_Poecilia_reticulata_Peters_1860
If the link doesn't work, the paper is:
Effect of Dietary Supplementation of Different Rates of Spirulina (Spirulina platensis) on Growth and Feed Conversion in Guppy (Poecilia reticulata Peters, 1860)
The author is Seval Dernekbaşı, and it was published in 2010
For the particular feeds they were testing, they got feed conversion rates of 2.31 to 1.09 (that would be how many pounds of food to get 1 pound of weight gain.)
Of course you might get very different rates if you used different foods, but at least those numbers can give some idea of what range you might encounter. The food is presumably dry, while the weight gain is presumably live fish, which contain a lot of water. If they tried to dry the fish, the ratio of dry food to dry fish would not look as nice.
If you are purely interested in cost, and if you have to buy food for the fish, it probably makes more sense to buy the fish food and feed it directly to the chickens as a protein supplement, instead of raising guppies with it. (That would also save the cost of fish-keeping equipment and buying fish.)
If you have a source of free food for the guppies, that could change the numbers quite a bit. I have read that guppies in outdoor tanks during the summer can grow nicely with no food provided at all, because they get enough natural food like mosquito larvae. I assume climate and other conditions make a difference in how well that would work.
If you want to have guppies anyway, I am sure the chickens would appreciate any extras and culls. (I haven't tried it myself, mostly because my guppy-keeping and chicken-keeping times have not overlapped properly.) I'm just not sure whether it makes sense (financially and in terms of time spent) to raise them specifically for the purpose of feeding to chickens.
If you try it, I would be quite interested in hearing how well it works!