Junior,
Many of the pen mandrels are not very concentric due to the fact that the end of the rod is threaded into the morse taper end of the thing. While this makes it easy to replace just the rod if it gets bent, it's virtually impossible to get a threaded rod to be concentric with the rest of the mechanism.
I haven't bought one yet, but I've got my eye on one of the collet type mandrels that you can pick up at the local Woodcraft store (mine's about 60 miles away) for about $25. If you subscribe to their newsletter, they're giving free shipping for most of the month...
In fact I just got a Nova Midi chuck with free shipping just last week. Too bad I didn't pick it up when it was on sale last month when I was up there...
Anyway the collet type is probably more versatile, (it will do several lengths easily) not to mention being more concentric than the threaded rods type.
If you have a barrel trimmer and a 7mm drill it should be a snap to make pen blanks... But to pay for blanks and have them be junk must be a real drag.
Of course, I'm speaking from no pen turning experience whatsoever, so anyone feel free to correct me.
I'm just speaking from general machining experience...