Fluid sprayed on to the airframe when the A/C is on the ground is v different to that which is exuded from a TKS system.
I know of a TKS deiced aircraft (not my own) whose TKS system is heavily contaminated with debris caused by using cheappie fluid. This is a very expensive and essential system to risk by using fluid which could be contaminated with particulate debris. Although the constituents may be present in the same quantities in truck deicer you may have some grit/debris in there which wouldn't matter if you were deicing trucks but would make a huge difference if your TKS stopped due to a blocked filter or blockage of the laser drilled holes in the titanium leading edge panels in heavy icing conditions.
Aeroshell 07 is a little cheaper in the channel Islands but still v expensive, it remains my first choice. (and I have used less than half a 20L drum this year so far).
SB