A quick comparison of retail prices show a price difference [between LGDs & naturals] of 20-40%, depending on the specific goods and the retailers’ branding, market positioning, etc. Wholesale prices behave very differently. In the wholesale market diamonds are priced as a commodity ... a much more accurate way of measuring price changes over time. Polished wholesale prices of LGDs are 50-85% lower than those of natural diamonds ... the smaller the goods, the larger the price difference. On average, 1-ct. lab-grown goods were priced 64% lower than natural diamonds at the end of December, while third-carats were 75% cheaper. If you are wondering why the price gap in retail prices is so much smaller, the answer is simple: retailers are keeping a wider margin.
Edahn Golan, from his diamond research blog, "LGD Prices – Not What You Might Expect"