I initially plan to look three places: the Sigma-Aldrich website, EMD-Millipore's site and also P212121, a site of Sean Seaver, a chemblogosphere stalwart. I have also added an unnamed lab supply house:
Sigma-Aldrich (4X4L, HPLC grade, ≥99.9%, 34851-4X4L): $1,150
Unnamed Very Large Supplier (4X4L, HPLC grade, 99.8% min (GC assay)): $190-230
EMD-Millipore (1X4L, HPLC grade, 99.8% min (GC assay), AX0145-1): $288.00*
P212121 (4X4L, HPLC grade, Purity >99.9%, PA-30000HPLCCS4L): $135.00**
*I know that EMD-Millpore offers 4X4L cases, but I can't find them on their public websites.
**There's a minimum of 5 cases.
Also, there is a University of Wisconsin-Madison datum that shows HPLC MeCN at $46.50 for 4L (thanks, Anon!)