A roasting rack and an aluminium baking sheet not only cost less combined than many roasting pans, in most cases, the combo works better than a roasting pan. Photo by J. Kenji López-Alt.
Serious Eats' J. Kenji López-Alt explains:
it allows for more even air circulation and transfer of heat energy to the bottom of the roast, resulting in more evenly cooked meat and crisper skin on your poultry.
[...]Even when you're roasting vegetables, an aluminium baking sheet will give you a faster cooking time, offer more space than a standard roasting pan, trap less steam, and be easier to handle.
There are a few instances where a roasting pan is the better tool for the job, such as for roasting vegetables below the meat. However, in those cases a disposable aluminium pan will do.
Check out Kenji's post for more details and buying recommendations for those of you who roast frequently and really do need a sturdy roasting pan.
Why You Don't Need an Expensive Roasting Pan [Serious Eats]