The feat is the only "cost" required. It lets you choose a single creature and apply the template to that creature. The creature type chosen must be a fey or outsider and available as a choice with the improved familiar feat. But the creature summoned is not itself a familiar, and wouldn't have the abilities and limitations attributed to familiars.
The creature can "level up" as you can cast higher level summoning spells, but that just keeps pace with the other creatures that can be summoned at that higher level. The feat is mostly just to give you a signature summon unique to your character, but it does have a couple of advantages. Since the creature is always the same creature, it can be useful for scouting, as you can have your summon infiltrate, and then just summon it again to have it report on what it saw. And the creature lasts for minutes per level, instead of just rounds per level, which is great for those who summon using spells instead of using SLAs, which tend to already have a minutes per level duration.
This feat is a pretty good choice for summoning-based clerics, as you can choose a creature that will always work with the Sacred Summons feat.