The only thing that really makes any sense is that scouts on San Diego think that Trammel is a lot better than his performance, that he's growing onto some sort of upside (MLB Trade Rumors calls Trammel a "much-hyped" prospect, whatever that means). And his draft pedigree is pretty high. His overall minor league RC+ is about 120, so it is possible he's better than he looks right now. Still a steep price given what I would think is his perceived value.
I'd add that Reyes' 27 HR/ 9 2B ratio is unsustainable and indicates a likely power regression for a hitter whose sole value is SLG.
One other note--even in this "bad" year Trammel's OPS+ and RC+ are over 100 (above average for league).