You get what you pay for....what size filter ring? I might have a circ pol you can have (72mm)
Otherwise, if buying new ones go for the best you can - the Hoya SFC Pro 1 is as good as they get, equal to B&W, Heliopan or any of the other vaunted makes and names. I just bought two new ones (smashed one, bent the other....) they were something like £140 each.
What do you get? Thinner glass with less refraction. Thinnner mounts so less chance of vignetting on wides. Proper coatings so you get true colours. Better polarising film sandwiched between the glass (which is how most decent polarisers are made - a thin film of polarising film stuck between two pieces of glass and shoved in a brass housing.) The ring will turn more smoothly, so you don't get that sticking feeling with graunching and grinding as you turn the filter.
ND grads - Cokin at £25 will likely give you mauve casts.
Lee ND grad at £35 will give you tonal neutrality.
Better filters are more expensive because they are....well, better actually. Yu can pay more, but the Hoya is very, very good if you get the Pro 1, they are as bad as everything else if you go for the cheap, uncoated one.
ND grads - how many do you plan on buying? So buy one good one, once. Forget the filter housing, you don't need it. A blob of blue tack does the same thing - save the cost of the filter housing and ring and spend it on the better filter! All the housing does is hold the filter in place - just like blue tack does!