hello friend
first of all battery cost is depend on your local computer market it differ from place to place depend on taxes vat etc.. added by sale shop on the product
normally there are 2 to 4 types of quality available in market original / oem / china /compatible. battery comes with 3 to 6 month warranty depend on which battery you purchase if you are looking for long backup then you have to go for 6 cell or 9 cell battery normally battery cone in 4 cell /6 cell /9 cell package . other details regarding battery voltage model number you have to refer label given on battery