So you're saying on the escort's ad it says $400 an hour but when you show up and tell her you're about to lose your 'V Plates' she charges you $450?
Doesn't sound like a likely scenario.
Who cares anyway - you're only a virgin once so you'll only have to pay the extra amount a single time.
Look, if you're inexperienced (not necessarily a virgin) you should pick an escort who is understanding of your situation. It's better to be up front about your situation as it will help the sex worker provide a service more catered to you. It might be worth seeing the same escort a number of times so that you get more comfortable and can build on your previous experience.