I was waiting for someone with actual experience with the Sunburst nibs to come forward, but I guess they aren't available presently. I therefore have only opinion to offer, not having any or ever have used any (I have lusted over one or two though).
I would think that they are fairly rare. I haven't seen any - well perhaps one on the 'Bay, and I would expect the price to be $20 - $30 for one. (just a guess).
Anyone out there with one MIB and willing to part with it for $20, just PM, and it's sold.
Gerry