My bid is way out of the budget you propose because I do understand how complex it is to make it actually right.
Convolutional Neural Network will need lot of tweaking to behave properly, and detecting face expression in a noisy environment, especially if you need OpenCV to tweak them out and detect it frontally. The price really depends on how much you need and your specifications, it could be lower.
Please provide me with details so we can discuss about it.