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Cheapest the 1866 has been on Amazon. Locally, the 4GB starts at $54 and the 8GB $85 plus delivery according to staticice.
All AU$ and includes delivery:
1800MHz
4GB $43 - Amazon's cheapest
8GB $73 - Amazon's cheapest
1866mhz 2x8gb in red colour is $147, so only $4 bucks more than the 1600mhz. Not that I could care less whether I had 1333 1600 or 2400 in my gaming system.
edit:
Isn't this slightly cheaper again?
Newegg.au - GSkill 1866mhz 16gb
$140 shipped