It's called supply and demand. I do domestic flights and it might be a bit easier to get people to VDB since we can put you on another flight within a few hours. The shorter the time between the flight I need you to get off from and the flight you will now be leaving, means less money that I offer. I tend to start low ($150) and offer more if I don't get any volunteers.

The lounges are not a tool that we are allowed to use. First class offerings are also rare.

Just remember you can also accept the initial offer of the VDB and should the GA need more volunteers and offers more you also get the higher amount.

$600 VDB for another flight in two hours is a great deal, you were silly not to take it.