You haven't mentioned your Category and the stream you want to study in IIST. Neverthless, the cut offs for various Categories in 2018 have been as follows:

This is 2018 data with the closing ranks. You can check the complete data at our page: https://engineering.careers360.com/articles/iist-btech-cutoff . Now cut offs change every year depending on various factors and so 50% might actually get you a seat in IIST. You can check the fee structure at: https://www.iist.ac.in/sites/default/files/admissions/UGSemesterFee.pdf It is approximately 50,000Rs per semester including Miscellaneous charges.