Go to https://play.google.com/store/account?purchaseFilter=apps, open the Inspector (CMD+ALT+I, using Chrome) and enter the following:
$('td.purchase-price').map(function(){ return +$(this).text().trim().replace('€', '').trim().replace(',', '.') }).toArray().reduce(function (memo, p) {return memo + p;}, 0)
To filter for specific in-app purchases, prepend a purchase name selector:
$('td.purchase-name:contains("Comics") ~ td.purchase-price').map(function(){ return +$(this).text().trim().replace('€', '').trim().replace(',', '.') }).toArray().reduce(function (memo, p) {return memo + p;}, 0)