Since there's always good interest in the Synology 4-bay NAS enclosures, the current pricing with coupon on the DS918+ is $615.20. The DS418play is $503.20 for those who don't need the performance benefits of the 918+.
Whilst I'm not entirely convinced on buying the DS918+ given it came out in November 2017 making it about two years old now, I'm sure the specifications and performance will still be plenty for most consumer NAS applications. I do note that the DS918+ has been cheaper before with other sales/Amazon deals/etc (you can check the history on OzBargain) but I haven't seen it below $600 lately so depending on your urgency, this might also be a consideration. If you're on the fence and also looking at other 4-bay NAS options from Synology, here's a convenient table I pulled together since I've been monitoring these since the last sale.
Alternatively if you are wanting a 5-bay NAS and wanting to spend (disproportionately) more, I have included the DS1019+ below so we can start to build up some history of pricing on OzBargain. This appears to be the cheapest price I can find today on StaticIce and eBay.
Again, for those who are curious about price movements vs the last sale in September 2019, here's the differences (note that DS418 Play might be the "value buy" this time around):
Unfortunately I do not have historical price details on the DS1019+ from the last sale as I wasn't actively researching it back then. Hope this helps.
EDIT: Just spotted the DS918+ marginally cheaper at $614.40 at Futu_Online with the 20% off coupon PLOTS20 ending 21/11 > https://www.ebay.com.au/itm/Synology-DS918-4G-4-Bay-NAS-Inte...
EDIT 2: Adding in the DS1019+ just so we start to have a bit of a history of pricing on OzBargain for the 5-bay model. Hopefully it'll get more heavily discounted in the future, but I guess at $919.20, it's still cheaper than everyone else on Staticice/eBay.