How much does "Resume from Summary" cost?

When resuming a large but old session, you are presented with the choice to "Resume from Summary (Recommended)".

But, I couldn't find any info on the cost on session usage. How much input/output tokens does the chat compaction after the TTL expires actually consume? Let's say I got a 600k Opus session which is expired. Will that just incur 600k of reading the stale session at Opus cost or does this summarization use Sonnet or Haiku by default?

Thanks for any pointers and/or empirical/experience on this.