Spirits are getting a huge support in Crimson Vow set, and Cemetery Illuminator is one of the most interesting new spirit mythics.
Besides the obvious choice for commander format, it is being viewed by modern players as a potential addition to the Azorius Spirits deck in the format.
The pre-release price for Cemetery Illuminator started at $25 and eventually dropped to $15. This means that the confidence in the card's value on the market has been decreasing.
It's really hard to say where the price will go, but it just looks like another case of a $5 card.