BEST BUY's low price guarantee doesn't explicitly exclude off-contract phones, but, it does exclude contract phones:
Low Price Guarantee - Best Buy
If a phone (or any product @BB) has the "LOW PRICE GUARANTEE" emblem (check on-line or in-store), you're good to go.
It still wouldn't hurt to try a price match/low price guarantee,whether on or off contract, as YMMV.
I prefer to purchase via BEST BUY, as I like their insurance better than everyone else's, but, this time around, I think I'm gonna get my next phone from T-Mobile, on their JUMP program, as the insurance is included w/the monthly JUMP fee of $10.