Get your cost basis from you Robinhood order history page at https://robinhood.com/account/history?type=orders

robinhood-cost-basis.js

/*
Scroll all the way down on https://robinhood.com/account/history?type=orders
Paste this into the dev console
`totals` holds your overall cost basis for each position you've held

Do note that this is just a quick bang out and likely isn't 100% correct
*/

const headers = Array.from(document.querySelectorAll('[data-testid="rh-ExpandableItem-buttonContent"]')).map(e => e.querySelectorAll('h3, h3 ~ span'))
const action_regex = /(.+?) (Market|(Stop )?Limit|(\$\d+ (Call|Put) (\d+\/\d+\/\d+))) (Buy|Sell)/
const price_regex = /(\$((\d|,)+\.\d+)|Canceled)/
const amount_regex = /(\d+\.?\d*)/

const reducer = (accum, val, currIdx) =>
{
    // first iteraction has accum set to the first item from `map
    // so reset accum to properly process that item
    if (currIdx === 1) accum = reducer({},
    {
        ticker: accum.ticker,
        action: accum.action,
        price: accum.price,
        amount: accum.amount
    }, 0);

    if (!val.price) val.price = 0;
    let price = parseFloat(val.price.replace(/[^\d.]/g, ''))
    if (!price || isNaN(price)) price = 0; // guards against "Canceled"
    if (accum[val.ticker])
    {
        if (val.action == 'Buy')
        {
            accum[val.ticker].price += price;
            accum[val.ticker].amount += val.amount;
        }
        else if (val.action == 'Sell')
        {
            accum[val.ticker].price -= price;
            accum[val.ticker].amount -= val.amount;
        }
    }
    else
    {
        accum[val.ticker] =
        {
            price: price,
            amount: val.amount
        }
    }

    return accum;
}

let cost_basis = headers.map(e =>
{

    action_match = e[0].innerText.match(action_regex);
    price_match = e[1].innerText.match(price_regex);

    amount = e[2]
        ? parseFloat(e[2].innerText.match(amount_regex))
        : 0; // no amount - Canceled

    return {
        ticker: action_match[1], // Disney
        action: action_match[7], // Buy/Sell/Canceled
        price: price_match[1], // 1,123.45
        amount: amount
    };
})
.reduce(reducer);

totals = Object.keys(cost_basis)
    .sort((a,b) => a.localeCompare(b))
    .map(key =>
    ({
        ticker: key,
        cost_basis: cost_basis[key].amount
            ? cost_basis[key].price / cost_basis[key].amount
            : 0
    }));