I recommend using a circular polarizer to reduce glare on windows and water as well as to enhance colour saturation in skies. As with any camera gear, you generally get what you pay for. Filters are no different. The higher the price point the filter, typically the better results due to better quality glass and coatings.. I would not spend less than $75 on a good polarizing filter.
I have posted a guide to using a polarizing filter and sample images in the bonus content section.
NOTE: The front of your lens has a threaded filter ring. It has a specific size which will be written somewhere on the lens. The filter you purchase, must match the thread size for your lens. Typical thread mount sizes are 49mm, 52mm, 55mm, 62mm, 72mm, 77mm and plenty of others. If you cannot find the filter size on your lens, you should consult the manual or google the name of the lens and the words filter mount size.
Reason: Cheap filters start in the $20 range and vary in price up to hundreds and even the thousand dollar range. For most of us, that would be overkill and frankly, not in the budget. The Promaster filters listed at this link are reasonably good quality and they have an affordable price point.
Price Range: Expect to pay $100-$200 for a polarizing filter depending on the size. (Oct 2018)