I would love to really help you out, however I haven't created an altcoin yet.
But let me do my best anyhow.

the cost can be 0
  - you just change some constants in the bitcoin code (or whatever code you're forking)
  - tons of coins were made like this. some even had some success (sadly!)

the cost can be a few man hours worth of coding
  - change the constants and change the PoW algorithm, with one that exists, but not yet in a coin.
  - tons of these exist, and it's a good idea to have alternatives to SHA2 in the cryptoworld

make an original PoW algorithm.
  - you'd better be a cryptographer though, or else it's probably not going to be secure
  - do any of these exist?


On the other hand, the costs can be quite high if you design a new coin from scratch.
   - nxt
   - ripple
   - CryptoNote
   - What else?
   - costs, revenue? ask them!


But keep in mind, that once you publish the source code you no longer control anything, so the revenue must be from collateral sources.
But unless you are designing a new coin, for forking coins it's a simple matter of advertising costs and returns. marketing, or whatever you want to call it.

So, generally speaking, the revenue will not come from you owning the coin, because you can't do that with free software, but rather from generating this new potential source of revenue.
But keep in mind that other people who haven't invested can exploit it too, and it's their right to do so, being free software.