There isn't a single "upper 50 percent average SAT score" for all Ivy League schools. Each school reports its own data, and the ranges vary. Furthermore, "upper 50%" isn't a standard metric used in reporting; schools typically report the 25th and 75th percentile scores. The 75th percentile would be closer to what you're asking for, representing the score above which the top 25% of admitted students scored.
To find this information, you need to check each Ivy League school's admissions website individually. Their common data sets often contain this percentile information. Expect to see scores in the very high range (1400s and above) for the 75th percentile on the 1600 scale SAT. Keep in mind that these scores represent the *admitted* students, not all applicants.