I see that many new Bitcoin 2.0 apps are making use of the additional OP_RETURN parameter for attaching data. I see that max limit is 40 bytes or roughly 40 characters? My question now regarding OP_RETURN is:
So is it one OP_RETURN parameter for each bitcoin transaction or per each input/output address? If its each transaction, so its safe to say its about .0001 btc to send 40 byte message?
Which desktop / mobile wallets support adding a custom OP_RETURN message? (ex. attach a short url link)
Aside from OP_RETURN message attachment, what other methods are there to attach custom data?