Webform Event Fees in Combination
Ah yes - you can do this without Conditionals! @Coleman showed me at the CiviCON sprint back in Ft Collins - Here's how in screenshots (below):

NOTE: the only downside is... that CiviCRM core isn't expecting that this is possible - so if you end up checking a participant record - it thinks you owe someone a refund [having paid $500 - the event was only $300]. But if you can make that clear to your admins - they can work with this. 

1. In Webform CiviCRM tab -> Event Registration:


2. In Webform tab -> hit Edit on the two participant fees - and add a $ number:


3. This is what it looks like for the user (View Webform):



4. And on the Payment Page of the Webform: