If they are adding water(and a few do, but not like your talking), it's not going to be a whole lot like less then 10% water...and most only do if they are using a lot of VG as it's a lot thicker then PG.
Water has a lower viscosity the PG/VG...so to much would make it just run out of the atty(horrible leaking, a little is kinda normal).
Water also needs a higher temperature to evaporate, so to much would make it not vaporizers correctly. (I'd guess it act like a flooded atty, until it ran out the bottom)
I dont think this something we need to worry about.
It's not the VG and PG that costs a whole lot in
eliquid.
By expense to make one bottle cost per ml for liquids... the Nic is the most expensive part, the flavoring, then the bottle, VG, and lastly PG.
If a supplier was wanting to cut corners, it be by lowering nic levels without labeling it as such.
Like selling 24mg...but it's really 15mg