Eh, I figured this isn't exactly a rules discussion so...whatever. If my fried brain chose the wrong forum, hopefully some nice mod will move this to 4e rules.
Anyway, the math:
1. For each level, I added together the value of an item two levels higher than the PC and the sell value of an item of the PC's level. Yes, I added the 100 gp for first level, just to be extra anal.
2. At levels 4, 5, 6, 9, 10, 11, 14, 15, 16, 19, 20, 21, 24, 25, 26, 29 and 30 I subtracted 4/5 the value of an item one less than the PC's level, to reflect selling off enhancement bonus items as the PC finds better ones.
3. But then someone pointed out that my chart was one level fast, so I dropped each value by one level.