～くらい in this context means "to the point where ～". びっくりするくらい最低 literally means "worst to the point where one is surprised", or simply, "surprisingly worst".
- 眠れないくらい嬉しい happy to the point where I can't sleep / too happy to sleep
- 目に見えるくらい大きい big enough to be seen
This ad is somewhat tricky because it plays on words using a double meaning of the word 最低. 最低 both means "worst" and "lowest", but when people just say 「最低!」 or 「最低よね!」, it's usually understood as a slangy expression which means "Disgusting!", "It sucks!" (If someone is surprised at something very cheap, they say 「安い!」 rather than 「最低!」)
In the first half of the ad, the two ladies repeatedly say 「最低よね!」 with an irritated voice, giving the audience the impression that they are clearly disgusted by something. Then this conversation follows:
A「でも…最低なのって…嫌いじゃないかも…」 But... being "saitei" may not be unfavorable...
B「最低って?」 Being "saitei" is...?
And the audience understands the true message of the ad, "Being the lowest (in price) is good!"