The bible contains many references to gold and its high value. Gold was one of the precious gifts that the Three Kings brought to the Christ Child, for example, symbolizing His kingship.
In the Old Testament, the Jewish people were instructed to adorn the new temple with much gold in anticipation of something greater. We are told in the Book of Revelation that that the Holy City, the Heavenly Jerusalem, is described by John as having streets of gold with a pure, translucent gold appearance overall.
Gold symbolizes the uncreated light in iconography. You'll see it used on backgrounds and halos to symbolize the heavenly realm. Icons and mosaic tiles are embellished with real gold which does not tarnish. A lit lamp placed before or near an icon will enhance its brilliance. Eastern style churches most notably, but not exclusively, make use of gold's illuminating quality.
The tabernacle in most churches is generally gold on the outside and lined with gold on the inside because this is the place where the Most Blessed Sacrament sits in repose.
Is it worth the cost?
Utilizing real gold is costly. When prescribed, however, we are reminded that a certain image or space is not ordinary, but set apart for the glory and honor of God and His saints and therefore most worthy of this adornment.
So, whenever gold is used in sacred or devotional art, it is in order to transmit anew this traditional and ancient connection of value and worth. Gold is not simply for embellishment, however beautiful it might be.