There are many choices of mylar bags available for purchase on the internet. Just go to Amazon.com and search for "mylar bag". You'll probably get at least 15-20 pages worth of options. Mylar bags vary in size, design, and thickness. Some are completely translucent, others have an outer paper layer. These variations contribute significantly to the utility and reliability for long term storage and also to the difference in cost. Just because a pouch is called a mylar bag does not mean that it is suitable for long-term storage and you should be careful in your bag selection.
Pleasant Grove Farm manufactures foil barrier bags (aka mylar bags) using a lamination process that yields a film consisting of 3 layers. The outer layer is made from BoPET (Biaxially-oriented Polyester). BoPET (Mylar) is a clear, translucent polyester film. Our bags have a middle layer of Aluminum metal (much like heavy duty aluminum foil). The inner-most layer is polyethylene plastic which is safe for direct food contact and allows for convenient heat sealing. Together these 3 layers comprise a high quality film that is 7-mil (177 microns) in thickness.
The thickness of the inner aluminum foil layer and the thickness of the film overall contributes significantly to the reliability of the bag for long term storage and also to the cost. A 7-mil mylar bag typically costs more than a 5-mil or 3.5-mil bag. Consider that when you purchase a 7-mil bag, you are getting a 40% thicker film than when you buy a 5-mil bag or 100% thicker than a 3.5 mil bag.
The lower cost of the thinner 3.5 mil bags is because these bags do not have a true aluminum foil middle layer. These bags have a brilliant shiny metallic appearance that comes from a sprayed / printed-on highly reflective metal looking substance that coats the BoPET film. These bags are referred to as metalized mylar bags. Some manufacturers and sellers of "metalized" bags attempt to convince you that these are superior to other 5-Mil or 7-Mil bags that contain a true aluminum foil layer. Hold these bags up in front of a bright window or place a pen light inside. If you can see light through the film, you have an inferior bag and these should not be used for long-term storage as one of the principal enemies of long-term storage (Light) can penetrate these bags and cause deterioration. Furthermore these thinner bags are highly susceptible to abrasion and puncture. Most manufacturers tell you the thickness of the bags they sell. Some avoid revealing that information. You should avoid buying from manufacturers that do not disclose the thickness of their film, or the material that comprises the film. There is a reason these manufacturers do not tout the thickness of their bags and actively attempt to conceal the film thickness and composition.
If you are cost conscious and cost is the only factor in your purchase decision, consider the cost and risk of loss of what you are protecting from deterioration over the long-run. You may save $ per bag right now by going with a thinner 3.5 or 5-Mil bag but if what you are protecting costs you $10, is it really worth saving a nickel now and taking a chance that when you come back later, you'll have lost $10?
7-Mil is the prudent choice.