It's cheaper because it's cheaper to make a striker fired gun than a hammer fired gun. The VP9 has fewer parts. I don't recall the actual count but I saw the parts list for both guns and it's a pretty significant difference.
As to which one is better...man, that's a tough one! I have a P30S with a trigger done by Gray Guns and a new VP9. I also have a USPc 9mm, USPc .40 and a USPf9 (as well as a USC carbine, and used to have a USP45 Tactical in .45). So I've shot a lot of HK guns.
Right out of the box, very first shot with the VP9 I hit dead smack on the X of the X-ring. I've never done that with any other gun! Subjectively it's as good a shooter as my tricked out P30S.
The bore axis of the VP9 is a bit lower; the hammer spring of the P30 runs down the backstrap. The VP9 has no hammer spring so there's room to undercut the grip more/higher up into the gun. In practical terms they're very close but he VP9 might run even flatter than the P30! The bullets seem to be self guided, and I was able to get good hits running the gun as fast I can pull the trigger.
It's very hard to pick a favorite. I too prefer hammer fired guns and I'm comfortable carrying my P30S cocked-and-locked. But I shoot the VP9 at least as well. It's about a wash.
The VP9 is a very good deal! I got mine for $599 with factory installed Meprolight tritium night sights and two mags. Big bonus- the VP9 and P30 use the same mag (which will also fit the USPc and P2000). I already had at last eight mags on hand so now I have ten.
I think the VP9 will be an epic product for HK. No one in their right mind should buy a striker fired service-type pistol without checking out the VP9.
Hopefully HK wil have a VP9c or VP9sk on the way before too long!
