Bioshock Infinite is likely going to be huge, so gamers who passed on the originals will want to go back and play them. Plus, new gamers are coming along all the time. It's been 6 years since the original came out. Plenty of younger and newer gamers may not be aware of it. And having everything in one case is a bonus in terms of perception. No worrying about platform exclusives or what DLC to get. All-in-one, bang. Done.
I imagine this will stick around. And yes, it will go down, but the original vanilla games still go for more than this, and that's without any of the DLC. So it's already a great price. There may not be an immediate need to discount it further.