I managed to find my cheaper solution. Although not as slick as these plugs, they do what I want them to do which is encourage growth of SPS and Zoanthids.
Its a sheet of 32 - (2 rows of 16) tumbled marble tiles that are 1"x1". Theres 2 types of tile per sheet, pure calcified marble, and quartz calcium marble mix. They are extremely pourous which should promote faster encrusting. Heavy enough to stop things from knocking them over on either eggcrate or loose in the tank. You do have to remove some glue however. I let them soak in RO water for 48 hours and let my kid peel the glue off 320 of them (which she thought was fun). I dropped all 320 of them into the sump 2 weeks ago and there is no effect on pH in the tank.
Cost from Lowes is $4.50ish per sheet. Cost per tile = ~ $.15
This isn't an exact pic of the tile sheet, but this is what the pure calcium ones look like:
