Taking into account the risks of mistaking a gray shade of the common olive green stamp for the slate error, or of purchasing a chemically induced color changeling, this stamp should be purchased conditional on obtaining expertization, optimally from M.E.P.S.I..
Given that caveat, however, the error is grossly undervalued, but will not remain so. It is priced at about 1/1,000th the value of an Inverted Jenny (U.S. Sc. #C3a), but is about half as rare (assuming that all of the 180 error stamps issued still exist). As the Inverted Jenny is one of the most famous errors in the world, however, perhaps this is not a fair comparison. The 20c Slate Color Error had a slightly lower printing than the U.S. 1893 4c Columbian Blue Color Error (Scott #233a), yet cats. at about 1/40th of the Columbian's value. To give some perspective on this, imagine a fantastical worst-case scenario, in which all 180 of C62a were to come on the market at once. They would probably be purchased for somewhat less than their full Scott value of
$ 90,000 ($ 500.- X 180) - less than a minor Mexican drug cartel makes on a slow day. The ludicrously low current valuation for this rarity is unsustainable.
With a population of about 109 million, Mexico has experienced consistent annual GDP growth of between 3 and 5%. It has a diverse and developing economy, but modernization remains a slow and uneven process, and current challenges include addressing income inequality and corruption, upgrading the infrastructure, and reforming tax and labor laws. Stamps of Mexico are popular among collectors in the U.S. as well as in Mexico, and those who wish to learn more about Mexican stamps should consider joining the Mexico Elmhurst Philatelic Society International (M.E.P.S.I.). MEPSI provides many useful services for collectors of Mexico, including expertizing Mexican stamps.