From the Fightin' Fourth: If you don't like the value which the presentence report claims the drugs were worth based upon the claims of a government agent biased against your client then you have to prove that the value was less.
Which I guess you would do by going out and surveying the local junkies? No, that would probably be hearsay. Maybe you could get a user to come in and testify as to what the price he's paid has been over the last few buys? Whatdoyamean, 5th Amendment? I'm sure that the prosecution would grant him immunity in the interest of justice and, of course, the judge would give heavy weight to his testimony.