As for the cost, Singh Ray and LEE are quite expensive. HiTech is quite reasonable. Tian-ya is quite cheap, but there is some issue with the colour cast.
You can refer the the following website for some of the pricing.
It is either "hard edge" or "soft edge" graduated filter. Soft edge has a gradual change over Get soft edge as it is more flexible in use. There are sizes for filters besides the brand name. Some say a 77mm lens needs the bigger size than the usual "P" size. I forgot what's it call.
Also graduated filters have either a 2 or 3 stop graduation.
Cokin and Hi-Tech (may be cheaper over there now) suffices for hobbyist.