You have to decided which 1 or 2 of these 3 factors are the most important to you: price, quality and effectiveness.
With price being the most important, you will be able to get a lot of material for cheap, but the quality of the materials will not be top notch, and you may not get the results you are hoping for. If you aren't happy with it, then you may have to apply another layer or 2, which will then multiply the cost significantly.
If quality and effectiveness are most important to you, then you might spend a bit more money initially, but your chances of being satisfied with the finished product increase significantly. There are well priced products that meet these criteria, Second Skin Damplifier Pro being one of them. They definitely aren't as cheap, as say B-Quiet or some of the other budget mats out there, but they are comparatively much thicker, and much easier/nicer to work with. However in the long run they will probably hold up better, and while it may cost more to begin with, you won't end up spending more money later to either buy more deadener, or to replace deadener that failed.
If price and effectiveness are most important to you, then you will probably end up buying large quantities of cheaper mat. If you do that, you might end up with twice as much material than you would have if you bought a more expensive, but thicker and better quality mat. In the end, you would be paying the same price for a lesser product.
If price and quality are your main concerns, then you really have to stretch your budget to accomodate your deadener purchase. This may mean holding off until you can save a few hundred more dollars, but ultimately you should be looking at the Second Skin line of products, which offer a great ratio of price to quality.
So sit down and do some thinking before you make a purchase, and decide where your priorities lie. Proper selection of deadener is important, as it is expensive and time consuming to install, and even moreso if you make the wrong decision and have to buy and install deadener twice because you are not satisfied. I ALWAYS recommend spending more money on a higher quality product, to make sure that you will be happy with what you get. I've spent thousands over the years on cheaper product, only to replace it later with better product, which ended up costing me far more than it woudl otherwise. These days I've smartened up a bit, and spend more money initially to get the better product. My wallet hates me, but my ears love me.