Typically £230 on eBay for the enclosure only.
Of course you will need to buy your own HDD's but these can be found on eBay starting around £50 for a 1TB drive.
Shipping is free (UK Mainland - Up to 7 working days).
This is the cheapest deal I have come across... Figured it was worth a post.
Now time for me to look for a POE switch & external IP camera. ☺