Right. Say the fight, as written, is very very tough. You, as the DM, figure the party can get through it, but just barely. They would expend a significant amount of resources and one or more of them has a chance of dying. You deem that this encounter will be worth 800xp per character.
Now your players, being canny folks, have their characters do research (spending time and resources). They purchase the weapon they will need to make the battle easier (spending resources). They construct a clever plan which, if successful, will not allow the enemies to even get in a single attack. This plan could fail - they are gambling with their lives.
Perhaps you could argue that this makes the combat too easy, so it is only worth 400xp now.
But you don't want to discourage all of this extra effort. After all, they spent as much real-time (and more game-time) working at making a good plan. Reward them. With another 400xp.
Total: 800xp. Just like if they had charged in.
To sum-up, the extra preparationg and planning should count for something. If anything adds to a characters life experience, it's discovering what plans work and what plans don't. Making a plan is worth enough XP to compensate for any added easiness in the encounter.
An example from my campaign. The party of 6 tenth level characters has broken into the lower levels of an old temple of Vecna. The rogue is scouting ahead (as all rogues should) and discovers a room where a collection of four wizards and four clerics are gathered around a table, performing a ritual to animate a homebrewed "Grave Golem." The party decides to ambush them. They make a plan to sneak up and cast all their area of effect spells at once in the room, hitting all the enemies. This is more risky than they know - four of the folks in the room are level 15 and the others are level 10. The PCs cast fireball, flame strike, and blade barrior simultaniously into the room, while the rogue throws a clay pot containing a stone with Silence on it. At any point they could have been heard or seen, but the enemies rolled poorly. The PCs rolled VERY well on their damage, and the NPCs failed all their saves. Only the high-level clerics survive, and the party barbarian and monk killed them before they could get out of the Silence.
This group of NPCs could, if things had gone just a little bit better for them, have wiped the floor with the PCs and been more or less unharmed. The PCs had real guts trying what they did. They got full XP for pulling off a daring plan, not some cheesy half-XP or something for going through all that risk for an "easier" encounter.