Carbon D. Metric
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I do not recommend EVER doing this kind of thing. The game is built from the ground up with certain assumptions about what kind of powers the PC have access to, and feats are only a tiny portion of the CR to a PC.
Any PC who chooses to lose experience in exchange for a feat will be losing BAB, Hit dice, Caster levels, Spell slots, Class abilities, skill points, and regular feat distribution.
If however you wanted to do something like this after my warning, I'd advise the PC would need to get at LEAST a full levels worth of experience (From their current level) before awarding them a feat.
Jal Dorak
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I would use the 3.X permanency spell as a guidelines. 100XP + 100XP x 1/2 character level should be about right (of course, adjusting for lower or higher XP progression as necessary).
Since this system would allow easier acquisition of feats, consider a minimum character level of 1 + 2x number of prerequisite feats.