PC Magazine 11/1984 was first reporting about the PC/AT 5170. They quote a price tag of 3800 USD for the low end configuration with 256 KiB and 5800 USD for the high end configuration with 512 KiB RAM and 20 MiB HDD.
PC clone manufactures to produce 80286 based AT compatibles would have taken at least until 1985. You can try to find the first ads in PC Magazine via Google Books...
The issue 12/1984 already has seller advertising the 80287 for 350 USD. At that time obviously intended as an upgrade for the IBM 5170.
By August 1984 Wave Mate was offering an 80286 upgrade board for the original IBM XT, starting at 1995 USD.
By the end of 1985 more XT clones with 80286 were announced. Still no AT compatibles, but obviously manufacturers were using XT tech to bridge the gap until true AT compatible mainboards became available. The Kaypro 286 reviewed in that same issue seems to be already AT compatible. It's entry price is stated at 3000 USD for the 512 KiB FDD only version.
Amusing side note: p136 talks about overclocking the 286 from 6 to 8 MHz...