On 12/21 EWR-SNA, I volunteered, and received $500.00 voucher. All flights that day and the next day were sold out, so they converted the volunteers' E-Tickets to paper tickets, and then "force booked" us onto subsequent oversold flights. When I checked-in for my 12/22 EWR-LAX flight, they were also soliciting volunteers, and the going rate was again $500.00. This time, I decided not to volunteer, but it seems like they will go from the starting point ($300) up to $500 very quickly.