(Note that in this example, I’m using a different product, because Agua Brava is apparently no longer available as I write this.)

Your example is on the right track. However, JavaScript uses zero-based indexing, not one-based indexing.

I verified that this produces a “missing value”:

tell application "Google Chrome"
    tell window 1
        tell tab 1
            execute javascript "document.getElementsByClassName('a-price a-text-price a-size-medium apexPriceToPay')[1].innerHTML"
        end tell
    end tell
end tell

I then replaced the 1 index with a 0:

tell application "Google Chrome"
    tell window 1
        tell tab 1
            execute javascript "document.getElementsByClassName('a-price a-text-price a-size-medium apexPriceToPay')[0].innerHTML"
        end tell
    end tell
end tell

And it returned the HTML code surrounded by that span:

<span class="a-offscreen">£19.35</span><span aria-hidden="true">£19.35</span>

To get the actual price, you would need, then, to get the innerHTML of one of the child elements of that span. Something like:

tell application "Google Chrome"
    tell window 1
        tell tab 1
            set productPrice to execute javascript "document.getElementsByClassName('a-price a-text-price a-size-medium apexPriceToPay')[0].children[0].innerHTML"
        end tell
    end tell
end tell
productPrice

This produces £19.35, which satisfies as a possible desired result in your question. If you want to do math on it, however, you would want to remove the £. This is most easily done by removing the first character.

tell application "Google Chrome"
    tell window 1
        tell tab 1
            set productPrice to execute javascript "document.getElementsByClassName('a-price a-text-price a-size-medium apexPriceToPay')[0].children[0].innerHTML"
        end tell
    end tell
end tell
set productPrice to characters 2 thru (number of characters of productPrice) of productPrice as string

Note that while this is setting productPrice as a string—which is necessary for gluing the characters back together because characters x thru y of string produces a list of individual characters—AppleScript is not strongly typed. You can do math on strings if they are easily convertible to numbers, as the string 19.35 (in this case) is. For example, if you need it to be rounded, you could use:

tell application "Google Chrome"
    tell window 1
        tell tab 1
            set productPrice to execute javascript "document.getElementsByClassName('a-price a-text-price a-size-medium apexPriceToPay')[0].children[0].innerHTML"
        end tell
    end tell
end tell
set productPrice to characters 2 thru (number of characters of productPrice) of productPrice as string
set productPrice to round (productPrice)

This produces the result 19, successfully rounding the string 19.35.

There are other ways you might get at the text £19.35. The span that actually contains the text has the apparently unique class a-offscreen, making possible the shorter command:

tell application "Google Chrome"
    tell window 1
        tell tab 1
            set productPrice to execute javascript "document.getElementsByClassName('a-offscreen')[0].innerHTML"
        end tell
    end tell
end tell

Or, it might (or might not…) be more reliable to get the nearest tag that has an id, which in this case appears to be corePrice_feature_div and then drill down from that:

tell application "Google Chrome"
    tell window 1
        tell tab 1
            set productPrice to execute javascript "document.getElementById('corePrice_feature_div').children[0].children[0].children[0].innerHTML"
        end tell
    end tell
end tell

You might even eschew getting text via searching for tags, and just run a regular expression over the entire body of the page:

tell application "Google Chrome"
    tell window 1
        tell tab 1
            set productPrice to execute javascript "document.body.innerHTML.match(/(£[1-9][0-9]\.[0-9][0-9])[^0-9]/)[1]"
        end tell
    end tell
end tell

This, also, returns £19.35, because “£19.35” is the first text that begins with a “£” immediately followed by a number from 1-9 that is immediately followed by 0-9, that is immediately followed by a period, that is immediately followed by exactly two numbers 0-9. Because this is a regular expression, you have a lot of flexibility in how you choose what to search for and what to avoid.

The regex match uses index 1 rather than index 0 because index 0 is the full match including whatever non-numeric character follows the price; index 1 is the first (and, in this case, only) parenthetical match.

All of these methods have the problem that they will fail when Amazon changes class names, or changes the layout of the page such that index zero is no longer the correct result, or starts adding more prices in arbitrary locations. Whether that’s a problem will depend on how often that happens, which you’ll find out once you start using your script regularly.

It probably isn’t worth worrying about ahead of time unless this is a critical app. Once you see how the page tends to change over time, you may find that one of the above solutions is better than the others, or that yet another solution will be more appropriate.