I can't give an exact amount of the percent without knowing what credit card you are using, what your
Best Buy rewards rate is and what discount you are getting the gift cards at.
But lets take an example of someone using the chase freedom and buying 2 $100 Mastercards with the Safeway offer:
$100 +$5.95 activation fee = $105.95 x 2 = $211.90 - $15 off = $196.90
Now, using Swagbucks at Best Buy you get 950 Swagbucks worth $9.50 but if it is the first payout for the month at Swagbucks you can get a discounted visa gift card so the $9.50 is really worth $10.92. You also get 5% back from your freedom credit card so another $9.85. And you also get 2.5% back from Best buy Rewards (assuming you are at the highest tier) so that is $5 back in Best Buy rewards which can't be used on gift cards. So for arguments sake I will value $5 in Best buy rewards at $3.
So your total cost would be 196.90-10.92-9.85-3=$173.13 for a savings of $26.87, or 13.4%.
Again, there are numerous variables that will change per person so this is a rough estimate. Some people may value the Best Buy rewards at the full $5 (although some people may not be at the 2.5% tier), some people may use a different card than the freedom which may earn more or less (for example the Amex Blue preferred earns 6% but has an annual fee which knocks the actual earning to 4.5% back but you may be able to get a sign up bonus for the first year.) Then again how much out of your way is it to drive to Best Buy (twice), assuming it costs the average person 40 cents per mile.
There will also be a deal at Giant/Stop & Shop next week for Mastercards getting 2X fuel points, but again, the value of fuel points are impossible to calculate as every person gets a different value out of them depending on your car and whether the gas station that you have to use is more expensive than others.
One more variable to throw out there: If the MasterCard being sold at Safeway or at Giant Stop and Shop is from MetaBank, it is not a problem but if it is a US Bank prepaid MasterCard these are highly prone to hacking and it might not be worth the savings to take the risk buying these.
To summarize, your savings at Best Buy should end up somewhere between 10 -15%.