Here are some links with information about how much the interest and penalties would be.
https://turbotax.intuit.com/tax-tips/irs-letters-and-notices/guide-to-irs-tax-penalties-how-to-avoid...
https://www.irs.gov/taxtopics/tc653
If you have filed an extension, and are struggling to figure out how to pay your taxes, consider filing by the extended deadline and setting up a payment plan arrangement with the IRS. This will eliminate a failure to file penalty, and reduce the failure to pay penalty by 50%.
https://www.irs.gov/payments/payment-plans-installment-agreements
You may also want to consider requesting a penalty abatement (removal of the penalty completely). If you usually pay your taxes on time but messed up for the first time this year, or if there are extenuating circumstances, the IRS may waive the penalty. Here's some info about that. You would wait to receive the notice from the IRS, and then follow the instructions to apply.
https://www.irs.gov/payments/penalty-relief-due-to-first-time-abate-or-other-administrative-waiver
