At the back office > Dashboard > Settings > General price settings you can set "Show prices: No" and no price will be calculated but the available cars will be still shown on the search results page.
You will have to also go to the back office > Dashboard > Translations > English and search and edit texts like: "See price", "Book now", "Grand total".
Also, please make sure that you only have Cash payment method published at back office > Dashboard > Payment methods.
The system will send you an email on each new booking and you can reply to your customer directly to negotiate the price.