In reply to The New NickB:
I could go into intricate detail (worked in the travel industry a long time though in the tech side now) but in short it's market forces. Published flights have staggered pricing across a range with generally tourist travellers getting higher fares the more seats get booked up, but the range does move upwards during school holidays as standard. Charter flights work similarly dependant on season and availability but are unconnected and the companies tend to have more flexibility as they own both hotel and airline.
You also need to take into account availability is linked to popularity - unsociable hours flights wlll tend to be cheaper, while packages may be based on flights (usually charter) which run at more sociable hours as they're selling more to families.
essentially if you're looking for a 'school holidays type trip' during the school holidays you're in a different market to someone who's not.
As for whether travel agents (as in nipping in the shop rather than self booking) are worth the extra money - they will charge more because the resources put in to making your booking are greater, and it's not just the hassle, it can be their expertise which, trust me, can be valuable. But they come in all shapes and sizes and there's ones that are pointless and there are ones that are brilliant.
The biggest way they make money I think at this point is simply that there is so much choice that people are overwhelmed. Going to a Travel Agent they will tend to have a set number of choices (even if it's just down to personal knowledge as opposed to available packages - it's likely they'll jump straight to options that they've been asked to push, have been to themselves, or simply that stand out in the system for some reason....oh and which ones are offering the most commission!). But having the choices narrowed can be helpful if you'res spoiled for choice so yo ucome out feeling like all the decisions have been made easier.