Yep! Easy to specify, hard to do!
You’re off by a factor of ten. 50m is 50,000mm, or 500 × 100mm.
I also would expect a very large price premium per unit length compared to the shorter blades, which must be made by the million.
I've thought about the price premium and think it could go either way. The blades from at least one manufacturer are already produced from a single ribbon before being separated, so the process to produce longer blades shouldn't be too different (hopefully!). And if there's a 2x premium, that's fine, if its closer to 10x I would re-evaluate.