in this context means **"to the point where "**. literally means " _worst_ to the point where one is surprised", or simply, "surprisingly _worst_ ".
> * happy to the point where I can't sleep / too happy to sleep
> * big enough to be seen
>
* * *
This ad is somewhat tricky because it plays on words using a double meaning of the word . both means "worst" and "lowest", but when people just say ! or !, it's usually understood as a slangy expression which means "Disgusting!", "It sucks!" (If someone is surprised at something very cheap, they say ! rather than !)
In the first half of the ad, the two ladies repeatedly say ! with an irritated voice, giving the audience the impression that they are clearly disgusted by something. Then this conversation follows:
> A……… But... being "saitei" may not be unfavorable...
> B? Being "saitei" is...?
> A "Great"!
And the audience understands the true message of the ad, "Being the lowest (in price) is good!"