B&H has the late 2015 iMac base model on sale (-$150)
http://www.bhphotovideo.com/c/product/1190385-REG/apple_mk462ll_a_27_imac_with_retina.html
If you just want the cheapest 27-inch iMac 5K and do not care about the slow mechanical hard drive, you could consider it (no tax in most of the states, that is ~$300 saving vs. from Apple store). PS: don't buy the 21.5-inch 4K model since the price difference is so small...
http://www.bhphotovideo.com/c/product/1190385-REG/apple_mk462ll_a_27_imac_with_retina.html
If you just want the cheapest 27-inch iMac 5K and do not care about the slow mechanical hard drive, you could consider it (no tax in most of the states, that is ~$300 saving vs. from Apple store). PS: don't buy the 21.5-inch 4K model since the price difference is so small...