The current cost of a theory test is £30 for car and motorcycle.
The theory test and hazard perception test for LGV and PCV are two separate tests. The fee for the LGV / PCV multiple choice test is £35 and the fee for the LGV / PCV hazard perception test is £15.
You can now book your theory test online https://www.nidirect.gov.uk/services/book-your-theory-test-online
The 1st part of your theory test is a random selection of highway code questions, the 2nd part of the test is a video Hazard Perception test, this will involve you clicking any hazards that appear on screen with a mouse button. Here is a example video of a Hazard Perception test.
Official example of the hazard perception test