cheapest on Newegg is $1299.
I imagine later this year, M.2 or mSATA will become more common place.
There is always this option:
mSATA adapters
Some laptops have an Msata slot.
Very cheap but outdated
http://www.notebookcheck.net/Review-Acer-TravelMate-P633-M-32374G50ikk-Notebook.96309.0.html
From manufacturer compatibility list
Notebooks & Tablets with mSATA slots
Acer: Iconia Tab W500, W700
ASUS Eee Slate EP121
ASUS Zenbook: UX21, UX31
Dell Inspiron: 14Z (5423), 17R, 17R SE
Dell Latitude: ST-LST01, XT3
Dell Precision: M4500, M4600, M6400, M6500, M6600
Dell XPS One 2710
Dell Vostro: 3360, 3460, 3560
Fusion Garage JOOJOO
Gigabyte U2442
Google: CR-48 Chrome
HP Pavilion: dm4t Series (DM4T-3xxx)
HP ENVY: 15 Series (15-3xxx), 17 Series (17-3xxx)
HP Folio: 13-1000
Lenovo: K26, K47A, K47G
Lenovo IdeaPad: S400, U300, Y460, Y470, Y480, Y500, Y560, Y570, Y580
Lenovo IdeaPad Yoga 13
Lenovo ThinkPad: X220, X230, T420s, T420, T430, T430s, T520, T530, L420, L430, L520, L530, W520, W530
Lenovo ThinkPad Edge: E220s, E320, E330, E420, E420s, E430, E430s, E520, E530
LG Xnote: P330
MSI: Windpad 110W
Samsung: Series 7 Slate, Series 9
Samsung Chromebook Series 5
Toshiba Portege: Z830, Z835
WeTab
(probably something good there at $600)
Edit: Look for laptops with SSD caching, Here is the dell 15z
http://www.dell.com/ca/p/inspiron-15z-5523/fs for example @$600. I believe that at that price range you are looking mostly at dell, lenovo and HP.