My chiller is a combination of 1/2in pipe up over the sides (plus one or two coils) and the rest is 8 metres of 3/8in. I made it with some scraps I had and some 3/8in that was given to me. The water flow through it was actually alot better than I thought and I'm very happy with it. I joined the 1/2in to 3/8in with some lead free solder (Aquasafe 100). At Tradelink they sell 3/8in copper tubing at $3.50pm, $5pm for 1/2in, however Bunnings sell 18metre rolls of 1/2in for $66, which equals $3.67pm.
Just some info. Good luck with it. Note copper pipe is measured by outside diameter, PVC piping and other stuff is often internal diameter (ID). Look out.
Cheers, Justin