Well if you are in the USA, Tower Hobbies has the 4PX w/R304SB for $550 but you can apply a $60 coupon discount code to make it $490 and then get the second receiver for the $550 price basically and get free shipping.

If you join the Super Saver club for a 1 Year Membership for $9.99 then that will open up a $75 off coupon code for initially add a $475+ item to your shopping cart and then have better value coupon codes for the next year for different spend points