school me on stroker smallblocks

A cheaper cast crank will have heavier and lightter areas throughout them. It's the casting process. Forging helps even that all out, and it makes the whole crank uniform density. That's one reason they are stronger. When one talks about "weight matched" parts, you have to understand what parts play what roles in the balancing of a V-type engine. Another thing is we are talking about rotation of the crank in the mains, and reciprocation (up and down) of the pistons, plus the mixed directions of the rod (both around and up and down).
The crank can be heavy or light in most areas because they are close to the mains and for the most part, the stresses are all different. It's all pretty much rotation. The crank also has torsional stress, which is twist as one cylinder fires, another compresses, etc. Torsional stress is what the harmonic dampner is designed to take care of. It just happens to be a good place to add counterweight for the factory desiners when the crank's counterweights are not big (heavy) enough. Forged cranks have more dense steel and therefore are heavier in the same physical volume than cast. This comes in when the cast cheaper cranks are used. Cranks are not balanced as part of manufacture.
The pistons have to be weight matched. They simply go up and down as part of the reciprocating assembly. Same with the rigns, piston pins and locks, and a little oil. The better forged pistons are close to matched. But there are always exceptions and errors. Cheaper forged or hyper or cast tend to be more wide in terms of weights individually. There's a reason they are less money. Wider tolerances, manufacturing, and lack of matching are part of the reason.
Rods are all over for a variety of reasons. I can show you set of "weight matched" Eagle or Scat or 440Source that weight the same. The issue is the piston pin end and the crank pin ends of the rods are considered different in the balancing process. The pin end is part of the reciprocating weight. The big end is part of bith the reciprocating and the rotating masses. So the overall weights are matched. but the ends can be all over the place (and ALWAYS are). Factory rods are that way too.
Factory assemblies can be internally balanced (with forged cranks and non-6pac rods) because the cranks are heavier and the strokes are shorter, so there is enough weight present to easilly bring things into balance within 30-50grams of so. Cast cranks need some extra mass just for the stock stuff, nevermind the longer strokes. So the factory used counterweights on the hamonic dampner, and the torque convertor. Again, it's a wide spec for "in balance". The B&M flexplate replaces the weight on the factory convertors so the aftermarket convertors can be run. No aftermarket performance convertor comes counterweighted for a factory engine assembly. Some will do it for you for extra cash.
I only internally balance because i want better than factory. Modern equipment can get the assemblies balanced to within .1 gram. One tenth of one gram. At 6K rpm that factory 30 grams of out-of-balance is pounds of force on the crank and mains of the block. The idea is to reduce the stress on the parts and allow more power to be made and available at the crank.
Costs will vary shop to shop. I pay $350 plus any heavy metal that's needed which can be pricey again, depending on your shop..