You're saying that 2x cpl doesn't work as a vari nd or just that cpl+linear is cheaper?
Two stacked CPLs won't work effectively as a variable ND. A CPL is a linear polarizer with a 1/4-wave plate behind it, that 1/4-wave plate in a sense 'un-polarizes' the light, because AF systems cannot use linearly polarized light. A varible ND cuts down the light as the orientation of a pair of linear polarizers approaches orthogonal (90° to one another would be 'black' if the polarizers were 100% efficient, which they aren't). The commercial variND filters have just one 1/4-wave plate at the back to circularize the the output so AF works. If you stack two CPLs, the light is already circularized when it hits the second linear polarizer (the 'front' of the second CPL), meaning you can't approach the orthogonal orientation that results in progressive light blocking.
Therefore, two stacked CPL's in 'normal' orientation (i.e. if you don't modify one of them) won't yield very much darkening. If you pull the glass from one of the filters, flip it, and put it back in the mount, and put that modified CPL in front of the unmodified CPL, then
it will work as a variable ND filter - but that's because by flipping it, you've put the 1/4-wave plate in front, and actually converted your circular polarizer into a linear polarizer. Easier to just buy a linear polarizer...
Bottom line, the way to make a DIY variable ND filter is with a linear polarizer stacked in front of a CPL (not the reverse).