about 15 to 20 bucks at most grow shops.....they have a light on them....then check the trichomes daily.....when almost all are milkey...and some are amber you are ready.....plants will mature at different times....I just harvested 8 OG Kypronite plants and it took about 14 days between the 1st one and the last one to mature.....here is when I took them 10% clear...60% cloudy....30% amber....you will always have some clear...as the plant is still producing trichomes.....Now this is only my opinion....some want mostly all amber trichomes.....In my case the 10..20..70 works for me + I needed the space to move in the plants that were vegging in another room....into the flower room.

Good luck