Quick answer is that bids seem to keep going up with bids over 20 now common.
yes
just the dollar amount.
This rule is part of the Axis & Allies Global 1940 2E, House Rules Expansion
For The Allies to be able - to some degree - to replace the severe losses of ships and transports during the early stages of the War, Landing Crafts are a plausible solution.
Though limited in numbers they still pose a serious threat for example to Axis-occupied Europe.
Landing Crafts are cheap, high value transportation means.