A mash is a liquid solution in which fermentation will take place. A mash is usually made in a vessel called a lauter tun. The basic requirements for a mash are that it contains sugars which yeast will be able to ferment, and nutrients to keep the yeast alive while fermentation is occurring.
There is a wide variety of sugar sources that can be used. Each sugar source will impart a different flavor to the final distillate. Such as:
Grain will make whisky. Blue Agave will make tequila. Apples will make applejack. Grapes will make brandy. Any mash or wash, (sugar, molasses, grain, etc) which will ferment can be used to make vodka.
Cost of mash
Of course your location will influence the cost of the starting ingredients in your mash, but there are some general trend. Usually mashes which contain just refined sugar and a nutrient for the yeast are the cheapest to make. Next, mashes made from grains are often slightly more expensive than sugar washes, however, grain mashes often require a large amount of work (especially if you are malting your own barley or corn). Finally fruit mashes are usually the most expensive mash to make. The noted exception if it is harvest time in a location with an abundant supply of the fruit, in which case the only cost may be that of yeast.
Sugar content in mashes
Grain mashes can often be made up to around 30% sugar (15% ABV final alcohol). Fruit musts are often comparable to grain mashes. For both, the maximum sugar content is also dependent on the grain or fruit that is used. Some grains contain much more potential sugars than other grains.