The cost of trips is not fixed. They get updated and can change continuously depending on availability and demand. This first and foremost happens at the airlines, but it can also happen at hotels. All trips are booked in a "live environment" and if a booking class on a particular flight runs out (many airlines have up to 16 different price classes per flight or aircraft) while you are looking for the trip, a slight increase of the price might occur. However, you always will be informed about this and you can choose whether you think it is within your budget. But just as the price can rise, the price can also go down if the demand is low.
We update the prices as often as we can, but it can happen that a price increases or decreases while you book your trip if someone else has booked before and the booking class is sold out.
This is entirely controlled by computers and the research by our suppliers, all in a live environment. It is more common that this will happen closer to the departure date, while it usually will not happen if you book well in advance (minor movements on the market). The world of travelling is a bit like the electricity market. Prices are changing according to availability and even according to the weather (bad weather = higher demand and prices rising = more mobile market).
We apologise for this, but we hope that the information above explains it well.