Buy cheap and you get garbage which leaves colour casts, that cannot be white balanced out. The Cokins are the worst you can buy, as well as their clones. The sky does not have a magenta cast.
All my landscape work utilises grads.
The least you would aspire to purchase are Lee or Hitech filters.
Then, it goes up from there. $200-$400 for 6x4" resin systems.
Filter holders are unnecessary, and, in some cases are detrimental to your image, leaving gradation bands in your images, particularly when using cheap stuff.
It's easy to handhold and dither the filter during the exposure, so you can lessen their effects, and make them less noticeable.
The goal is to create an exposure which looks natural, and, not be obvious a filter was used.
For example,
http://users.tpg.com.au/octane2/oms2.html was one where I used a 3-stop filter and its use is unnoticeable.
H