Like said above: cheap won't get your GND's. Color casts are your biggest problem, but thankfully, those can be easily corrected in LR (or other PP apps).
Most cheaper (G)ND's have a magenta color cast, which is super easy to fix.
I can recommend Cokin filters. They're cheap, versatile and are good value. You want to look at the Cokin P-series (I think up to 82mm filter thread? More than big enoguh). You can get a 0.3, 0.6 and 0.9 GND filter set (including holder and other accesories) for like 75$ from B&H. It's a good starting point. You just have to buy a filter ring to adapt the system to your lens and you're set. You don't have to buy a Cokin adapter. Just get a cheap one from ebay.