Butterfly is a decent on-line retailer -- I have purchased from them a few times and always received good service. That price sounds about right. Check this link out for a recent pricing survey:
Pentax K-x stock status and availability matrix - 1001 Noisy Cameras
As others have commented, the 55-300 zoom is considered by most to be a much better lens than the 50-200 zoom. Since the Christmas sale madness, K-x kit prices have increased, but you can still find the two lens kit (with 55-300 zoom) for under $700. I recently purchased this kit from Tri-State Camera & Video for a total cost of $684. (The same package had been going for as low as $650 earlier in the year.)
Mike