How many weight units does an input use? What is the weight unit cost of an input? And can you batch/aggregate the inputs or is it linearly growth when you add more inputs to a tx?
What is the weight unit cost of an input?


2 things affect the weight of a tx, the total number of bytes , and the total bytes minus witness data.

Transaction Weight = Base transaction size * 3 + Total transaction size, where

Base transaction size is the size of the transaction serialised with the witness data stripped.

Total transaction size is the transaction size in bytes serialized as described in BIP144, including base data and witness data.

See BIP141


  Can you batch/aggregate the inputs or is it linearly growth when you
  add more inputs to a tx?


You can't batch inputs, but you can combine them and send them back to yourself first in a larger amount, but that requires an additional transaction. Probably the best you can do now is to use a segwit address to receive funds so when you go to spend the inputs, the witness data lowers the weight of each input.