var elValue = "-12,erer3 4,-990.234sdsd";
var isNegetive = false;
if(elValue.indexOf("-")==0) isNegetive=true;
elValue = elValue.replace( /[^\d\.]*/g, '');
elValue = isNaN(Number(elValue)) ? 0 : Number(elValue);
if(isNegetive) elValue = 0 - elValue;
alert(elValue); //-1234990.234