Have new (used on one roll to check optics) 18mm T Distagon...want 21mm T Biogon. The only thing I can go on is completed listing at feebay, and it seems (at least recently) the 18 is fetching about 1300 and the 21 about 1000. Though, what's up there now are some sales of new 18's and used 21's so not quite fair. Anyway, long story short...unless I'm off-base, this seems like a straight-up trade. As the 18 I have is - basically - new, I'd want something with low mileage as well. I have 15...so my super-wide is covered...so am looking at the 21 as my 'tweener until the 35.