If you still want the soundproof effect without ponying up for the theater curtains, there are several tips for getting a quieter room.
First, you can try to get regular curtains simply made of thicker materials, which can actually help a lot if the noise is not too bad.
They also offer soundproofing materials, so you can buy however much you need, be it less or more than a curtain, and tailor your soundproofing to your room.
Finally, if the noise is really bad, then you may want to go beyond soundproof curtains. The same companies that sell the soundproof curtains also offer soundproof windows and panels.
The panels can be a particularly good option for apartments, since the source of the noise is often through the wall, rather than the window.