NPCs should not be built the way that PCs are.
It's impossible to say how much XP such an encounter would be worth, because PCs and NPCs aren't balanced the same way.
Generally speaking, PCs are glass cannons while NPCs are damage sponges. Daily powers in particular tend to be much more potent than anything NPCs of the same level have access to, precisely because they're intended to be able to deal with multiple NPCs. Likewise, PCs tend to have more HP than their opponents, especially if they have multiple ways to spend their larger pool of healing surges (such as second wind and a paladin's lay on hands).
These factors combine to make it extremely difficult to assign an XP value to an NPC built as a PC, because the range of what could happen is so much wider. Your paladin NPC might lose initiative and go down in two or three good hits without ever accomplishing anything, because he has less HP than an elite of his level. Or he might win initiative, use his daily power to drop a single PC, bloody several of them, or set up a zone or other lasting effect that gives him a large advantage, and then use his healing surges to stay in the fight far longer than any foe from the Monster Manual could because the weakened players are unable to finish him off, ultimately being almost a solo of the party's level. Even at level 3, with a single encounter power and daily power, it's hard to estimate how effective a PC will be against other PCs, and at higher levels it becomes even worse.
When you want a custom NPC, your best bet is usually to look through the Monster Manual and find something with the right combat style at about the right power level and then completely re-fluff all of its abilities. Need a powerful wizard? Use a beholder and describe all the eye-beam attacks as spells. For a paladin you should be able to use a level-appropriate soldier; one with some combination of the leader sub-role, healing, and marking would probably work best.
4e takes a very narrativist approach to character stats; rather than giving characters stats appropriate to their role in the world, it gives them stats appropriate to their role in the story that the DM & players are telling collectively. That's why an ogre can be an elite or even a solo in the low heroic tier and then that very same ogre might be a minion when the players are in the low to mid paragon tier; when the ogre was the big bad guy he had the stats of a big bad guy, and once he was just a lackey he had the stats of a lackey. Thus, it doesn't make sense for an enemy lieutenant of middling importance to have the same stats as a hero who fights a couple dozen monsters every day.