In an email, Unity declared that a $4.99 minimum price will be required for all new assets starting November 1, 2018. Free assets will continue to be free. On January 1, 2019 all store assets (except free) will be changed to the $4.99 minimum.
Unity says the change will be beneficial for all parties:
Currently publishers are free to price their asset at nearly any price. This has resulted in a lot of freedom, but also a fair number of odd prices, including many assets for very low prices, as low as $0.05.
These low prices don’t make much sense for customers, for publishers, or for Unity, as we lose money when someone buys these low price assets.