A late fee under Section 234F applies: up to ₹5,000 if your total income exceeds ₹5 lakh, or ₹1,000 if it's below that threshold. On top of that, if you owe any tax, interest under Section 234A accrues on the unpaid amount from the original due date until you actually file.
The bigger cost is often not financial: a belated return generally can't carry forward certain losses (like capital losses) to future years the way an on-time return can, which matters if you've had a loss-making year in the markets and wanted to offset future gains against it.
There is a hard outer limit — belated returns typically must be filed by the end of the relevant assessment year, after which filing becomes considerably more complicated. If you've missed the original deadline, the practical move is to file as soon as possible rather than waiting, since both the late fee and interest are calculated based on how late you actually are.