I have heard people say many times over that the value of a product is on its quality. With that being said the features of the mallet and their quality should be considered before making a purchase. These are a few things I might look for when buying a mallet.
1. Heads: Solid wooden head. Many players don't know this but there are many different kinds of heads in the ancient game of polo. You want a little density to create a hard snap on the ball but not so much to make the mallet heavy.
2. Shafts: You will have to decide if you want a composite or a natural shaft. Personally I like natural shafts for the play and feel. Some games may require one over another. Be mindful of your local rules and buy what make sense to you.
3. Length: The mallet should be able to reach/near the ground for easy hitting of the ball. 52" is very common for a size but there are times when a 53" or 54" might be helpful if the horse is larger.
4. Construction: Look for quality work. There are some lower quality versions you can get that have all of the beautiful stickers and other dressing type enhancement. Look past all that into the construction of the club and its quality from handle to head.
5. Company: If your going to do business with a company try and do so with one that supports good corporate citizenship. Personally, I prefer American companies but there are not many American producers.