Set the Webform-CiviCRM Contribution Amount to a parameter value
Just adding to this -> this is now possible (since today: many thanks for Fuzion for re-asking this question and helping to see it through!); we had to relax a form_error that was preventing tokens to be injected in the default field for
contribution_total_amount;

You can now do: 

/webform-civicrm-example?civicrm_1_contribution_1_contribution_total_amount=120


and if you have configured your Contribution Amount -> default as:

[current-page:query:civicrm_1_contribution_1_contribution_total_amount]


Then it will fetch that $120 -> and give you: