I believe the computer system tells the agent what the voluntary bump value of the paid ticket is, plus the $100 or $300 depending upon length of delay. My understanding is the bump value of the ticket purchased is the base fare paid plus the Federal Excise Tax.

I volunteered on my last flight, an award flight, but wasn't needed. However, I did ask what value (if any) would be assigned to my points ticket and after hitting a few keys, the gate agent told me the dollar value of the points ticket I had and said that amount would be added to the $300 for the 7 hour delay.