What should the amplitude be when plotting 1-sided Amplitude Spectrum?
To complement @wrapperapps answer, you can express sinusoids as sum of exponentials: $\cos(2 \pi f_0 t) = (e^{j 2 \pi f_0 t} + e^{-j 2 \pi f_0 t})/2$.

Thus, the fourier transform yields $\delta(f+f_0)/2 + \delta(f-f_0)/2$: each frequency delta has amplitude $1/2$.