How to upgrade my account for a short period

I would like to upgrade my zoom account to accomodate over 100 people for an online webinar.   When I go to the next level up it says that i must take out over 10 licenses which makes is super crazy expensive  The other option seems ot be to pay $50 for a one month and pay that each Month.     Since I only want about 150 people that seems a very expensive thing to do.  How can I accomodate more people in a cost effective way?

Thankyou