var numLines = item.getLineCount({
    sublistId: 'price'
});
var yourPrice;
for (i = 0; i < numLines; i++) {
    var priceLevel = item.getSublistValue({
        sublistId: 'price',
        fieldId: 'pricelevel',
        line: i
    });
    if (priceLevel === 'Your internal price level id') {
        item.selectLine({
            sublistId: 'price',
            line: i
        });
        yourPrice = item.getCurrentMatrixSublistValue({
            sublistId: 'price',
            fieldId: 'price',
            column: 0
        });
    }
}