I can't give you a pat answer on the dollar amount, but the Honda dealership can.
100,000 miles? My guess is ,that year, it's way past due for sure and yes, that would be necessary right away.
They'll want to do the water pump too. $1,000.00 total bill maybe.
My guess is that is an 'interference' engine. Meaning when the belt breaks there's interference between the valves and pistons and that will be COSTLY to say the least!
If that happens, consider it totaled for all intense and purposes.