I have an NZXT Phantom with a 200mm fan slot available and am using an i7 4770k. I really want to overclock so it must be a pretty decent cooler. I am looking for a watercooling heatsink below $50. I am also wondering if a dual 120mm radiator can be mounted on the said 200mm slot.
For $50, water-cooling in any form shouldn't even be a consideration. Quite simply you only have low end CLC's available to you at that price, and they are not worth buying.
Get yourself a decent air heatsink like the CM Hyper 212 EVO, unless you plan to go over ~4.4Ghz then you should be fine.