Solids are a lot cheaper!!
I assume you mean solid flat tappet vs any roller (hyd or solid).
Roller setups require a roller cam, roller lifters, bronze gear on dissy shaft etc and they don't come cheap.
The roller lifters allow for a much steeper lobe rise. Flat lifters just cant hack the geometry. The steeper lobe rise means more duration at the higher lobe lifts (lobes are much fatter at the top).
See pics at bottom of link.