The price of any ticket can go up or down in todays world. Some tickets are refundable and are in effect an option to fly. Most are a deal you do with the airline at a price that you both accept.
Silverjet are a startup with just one flight each way LTN-EWR. They also have had tech problems with the one plane that operated to route so will be losing some repeat buisness. The shareholders will need to see more and more people travel each month as it builds trade. As a result they will have to discount tickets just before departure if the plane is less than half full. It is your choice to book ahead or take a chance.
August will be a big test as they try to go to two filghts each way. That means the need a lot more bums on reciling beds. It is not going to be easy for them to get a good load factor and keep a good margin on each seat sold. The big chalange will be avoiding more breakdowns. They can take many days to catch up after each problem. The second plane will allow them to pull one and recover quickly.
I suspect Silverjet just started at the wrong time and could have worked very well if it had been started 5 years ago. Silverjet has got a heap of cash from investors to burn over the next 18 months. Then I expect it will merge into another small airline.