"Need" is a troublesome word. If you like the sound you are getting without a subwoofer, you don't need one.
However, a subwoofer could add the low frequencies that you are not going to be getting with your current speakers, and you might like that.
If I were in your situation with the budget you have, I would wait and save my money until I could afford a really good subwoofer, as cheap ones don't tend to be satisfying in the long run.
If you want accurate sound, what you want in a subwoofer is a flat frequency response (i.e., not overemphasizing or underemphasizing any frequency), and the ability to play very loudly with low levels of distortion. For this sort of information, one looks to measurements of the particular subwoofer in question, preferably by an independent party who has no vested interest in whether you buy one subwoofer or another.