I broke one of the rocker arms on my B7100 once, and ordered the replacement right from the dealer. Was expecting to get just one arm, and instead got a full kit with both arms, shaft, and all hardware. Maybe Vic knows, but it's almost like they made a change that necessitated replacement of both sides, or had problems with these breaking?? Don't know, but I was pleasantly surprised that I got all that for about $80. Actually, I only used one arm, which fit OK on the original shaft, so I've got a back-up, if it ever breaks again.
Assume yours might be similar on the 6100, so check dealer - it might not be all that $$$.