godan verb (-ru)
decline in price; fall in price
Intransitive verb used when prices go down. Often used for market prices, stock prices, or the cost of goods.
ガソリンの値段が値下がりした。
The price of gasoline fell.
株価が大幅に値下がった。
Stock prices declined sharply.
Transitive counterpart meaning 'to lower a price' or 'to reduce a price'. 値下がる is intransitive (prices fall by themselves), while 値下げる is transitive (someone lowers the price).
General intransitive verb for 'to go down' or 'to drop'. 値下がる specifically refers to a decline in price or monetary value.
Compound of 値 (ne, 'price') + 下がる (sagaru, 'to go down'). The reading ねさがる is a straightforward combination.