There policy is to price match plus 10% of the difference deducted.
Example: Best Buy price is $25 and you PM for $20 they would give you 10% of the $5 difference making it $19.50. Thus beating the price.
Though most will just PM for the exact amount. But I have never heard (or read) about getting 20% off.
Here's the post on Best Buys website:
Best Buy Retail Store Price Match Guarantee
If you find a lower advertised price on the same available brand and model prior to your purchase or during the exchange and return period, we will match that price. Simply bring in the ad of the local retail competitor or Best Buy, while the lower price is in effect and receive your price match. The guarantee does not apply to our competitors' website pricing and the guarantee does not apply to our or our competitors':
Offers that include financing, bundling of items, free items, pricing errors and mail-in offers
Items that are limited-quantity, out of stock, open-box, clearance, Outlet Center, refurbished/used items and items for sale Thanksgiving day through the Saturday after Thanksgiving
I worked at Best Buy and we only would PM. Now I have seen and heard of other stores doing 10-20% extra. I have also gone to different BB to price match items myself and have only had the PM. Good luck! YMMV