Plenum rated cable has a special insulation that has low smoke and low flame characteristics. Plenum cable is mandated to be installed in any "air handling" space. For example, most large office buildings use the ceiling to return air to the AC unit. This qualifies this ceiling as a plenum ceiling, and all the cables that go through that ceiling must be plenum rated. Please check your local building codes to see if you need plenum cable. The reason why it costs so much more is because the material required for the insulation must meet the standards for plenum cables while also meeting the standards for category 5e. This material is relatively expensive.
For your plenum applications, L-com carries: