A lot depends on the actual material. $140 could be very cheap or quite overpriced. There are many green, emerald-like stones. Most are "imitation", some are "synthetic". What type of stone are you being offered?
the stone I was being offered is the square lab simulated emerald stone that has 4 corners tapered off like the Radiant cut stone, and color is very much like the real emerald stone.
I see lab simulated emerald, rubies and sapphire stones in mall, and I can tell lab from real ones that lab created ones are w/out any inclusions. Real ones tend to have inclusions.
so what do you think? I saw on web that they sell lab simulated emerald stone for very cheap too. On this forum, someone asked about lab simulated sapphire, and answer was $5-$10/carat!