Flat-Bottom
Flat-Bottom letter molds are the most inexpensive for two reasons.
First, they are simpler to make than the V-Bottom letter molds. (It
can be a little tricky figuring out what angle of bevel will work on a V-
Bottom mold.)
Second, they can often be made with thinner stock than the V-
Bottom letter molds.
In fact, with larger letters, sometimes a Flat-Bottom letter is required.
The reason being that in order to create a V-Bottom the depth of the
letter may be too great. For example, in the diagram on this page we
see a section of a letter that has a six inch wide stroke (letters 18" and
up can easily have segments this wide).
As you can see, if a V-Bottom letter was desired, the overall letter
depth would have to be close to 2" deep, (even with the lower-angle
bevel shown in the diagram). This depth may not be acceptable as it
may weaken the structure. Also, to get that depth in the V-Bottom
letter requires using thicker sheets of Duro-Tek, which adds
considerably to the cost. On top of that, in order to cut a bevel that big
requires either a very large (scary) bit or muliple passes. Either way it
means more expense.
So, since the Flat-Bottom letter molds are easier to make and can be
made with thinner sheets of Duro-Tek, they are often the least
expensive way to go.