Signal Before a Binary Trade

A trade pays +8 in a favorable state and -5 in an unfavorable state. The favorable state has prior probability $\frac{2}{5}$. Before trading, you may buy a signal for cost $\frac{1}{2}$; it is correct with probability $\frac{4}{5}$. If you see the signal, you may either trade or abstain after observing it. What is the value of the signal, and should you buy it at that cost?