Using the bitcoin client's JSON API (or any other altcoin client) how do I find out the reward given for finding that block?
The information you get for a
getblock call doesn't seem to include the block reward. You could find it with a
gettransaction call for the first transaction, but bitcoind only allows lookup for transactions in your own wallet, so that won't work either.
So indeed, as Diego points out, the only option is to look at the block height. This wiki page lists the different reward values for the different block height intervals.
All you need is the block number. The reward started at 50 and it halves every 210k blocks. Currently it's at 25, and as I write this the current block number is 286459. For other altcoins, you need to know the schedule.