Yale: 2~3% (20~30/1000)
"Yale receives more than 1,000 transfer applications each year, and we have spaces for only 20 to 30 students."
source: http://admissions.yale.edu/transfer
Princeton: 0% (Does not accept any transfer students)
Stanford : 1.9% (33/1764)
source: http://ucomm.stanford.edu/cds/2014#transfer
MIT: 3.4% (18/535)
source: http://web.mit.edu/ir/cds/2015/d.html
Columbia(CC and SEAS): 6% (150/2500)
Q: How many students are admitted as transfers?
"Due to the high retention rate in both Columbia College and Columbia Engineering, there are usually very few spots in the sophomore and junior classes. As a result, we typically admit fewer than 10 percent of those who apply for transfer admission each year. Though the number varies from year to year, in recent years, we have admitted roughly 150 transfers from over 2,500 applicants."
source: https://undergrad.admissions.columbia.edu/ask/faq/topic/393
UPenn: 10% (200/2000)
"Each year, about 200 students transfer into Penn’s four undergraduate schools, selected from a pool of approximately 2,000 transfer applicants."
source: http://www.admissions.upenn.edu/apply/transfer-admission