I think the question here is how inexpensive are you talking about?
And how much of a line array are you interested in doing.
I mean a real line array must be able to be coupled to the ceiling and the floor, which means that the array must be roughly 70% of the height of the ceiling to floor. For example, 8 o of the Vifa's are not going to be long enough to couple with the ceiling and the floor, and if you don't provide coupling your array might not be long enough to guarantee listening in the nearfield. Listening in the nearfield is very very very important to the line array experience. Otherwise you just have a larger point source experience, but not a line array.