Hmm, it was more convenience than cost.
From a drug store (we call em chemists) it was about $1.80 for a litre. my system holds about 3L. I had only bought 5L, and was using an auxillary 30W pump to help me fill the system. The water was flowing fast and furious, splashes etc, case had to be open so i could have access to the PC's resevior and pump so i could shake it to get the air out
After i got it all flushed out and filled up and the air bleeded, I just thought "f* this. this is good enough"