How much does it cost to fork bitcoin?
The cost to
fork Bitcoin is free. One merely needs to make an incompatible change to the software that will fork the network.
The cost of a Bitcoin fork (like Bgold) is externalized onto the ecosystem. Newbies get scammed by downloading malware wallets or naively inputting their private keys into nefarious web wallets. Exchanges and wallet services need to spend endless hours building new wallet infrastructures to safely handle coins on both chains. This is particularly true if the fork has no replay protection. And if there is no replay protection, there are potentially huge losses for users, both because of individual mistakes (not splitting coins) and custodial mistakes (exchanges/wallet services fucking up and losing coins).
So, it's free for the fork devs. And there might be "free money" in it for those who can properly secure their money and split their coins. But there are certainly costs involved.....
The devs that decide to pre-mine the fork (like Bitcoin Gold) definitely get a nice premium. Some devs can also know what exchanges are going to list their fork before the big public, which means they move their BTC to these exchanges to have instant liquidity to dump the fork for BTC if want to, and in general manipulate the price. A lot of price manipulation is going on on all forks, BCash mostly, and now Bitcoin Diamond too.
It can be considered free money tho... as a BTC user, you don't really lose anything, other than time securing you access the coins properly and the fees, but if you do it at the right time, you can find profit on that.
The only scenario where it is not free at fundamental levels is when the fork uses SHA256, because the fork competes against BTC for the hashrate (which is why BCash is an attack on Bitcoin).