How can someone calculate the current/real-time value of a Bitcoin block?

Steps:

1. run bitcoin-cli getblockcount to get current block_height
2. current_block_reward = 50 / [(block_height MOD 210000) + 1]

What is the equation/algorithm used to determine this?

Value the of a Bitcoin block defined here. The default value is 50 * 100,000,000 Satoshis.

After every "SubsidyHalvingInterval" blocks, the reward of each block halved. The SubsidyHalvingInterval is defined here

CAmount GetBlockValue(int nHeight, const CAmount& nFees)
{
    CAmount nSubsidy = 50 * COIN;
    int halvings = nHeight / Params().SubsidyHalvingInterval();

    // Force block reward to zero when right shift is undefined.
    if (halvings >= 64)
        return nFees;

    // Subsidy is cut in half every 210,000 blocks which will occur approximately every 4 years.
    nSubsidy >>= halvings;

    return nSubsidy + nFees;
}

You can also use the BlockExplorer API totalbc query: http://blockexplorer.com/q/totalbc/####, where #### can be a past or future block number, and the return value is the total BTC in circulation. bcperblock may also be helpful.

See BlockExplorer.com/q/ for a full list of queries.