For eBay Plus users Shaver Shop appears to offer the lowest $/head with the 8 pack, express delivery included.
Unsure about the non-Plus users price and delivery, but Shaver Shop have always been blazing fast with their deliveries.
Don't be like me, who bought this a couple of days ago for a higher price and get it now that it's further discounted.
In case you are wondering, yes, it is likely compatible with your Sonicare brush (as long as it has a push-to-fit head with a kind of D-shaped metal rod sticking out of the main body). Yes, it is also compatible if your brush does not have the wireless function thingy.
Thanks to RogueWolf for his comment about the mixed pack with a slightly cheaper price/head:
Sonicare Optimal Brush Head 6 Pack(ebay.com.au) $39.95 with $5 off monthly voucher
Pretty good price. Roughly same price as 6 heads at Costco.