Get the price with tax

That's indeed the function you should use.
$productClass = hikashop_get('class.product');
$product = $productClass ->get($id);
$currencyClass = hikashop_get('class.currency');
$products = array(&$product);
$ids = array($id);
$config =& hikashop_config();
$main_currency = $currency_id = (int)$config->get('main_currency',1);
$zone_id = explode(',',$config->get('main_tax_zone',0));
if(count($zone_id)){
$zone_id = array_shift($zone_id);
}else{
$zone_id=0;
}
$discount_before_tax = (int)$config->get('discount_before_tax',0);
$currencyClass->getPrices($products,$ids,hikashop_getCurrency(),$main_currency,$zone_id,$discount_before_tax);

Then, the prices will be in $products[0]->prices

getPrices returns all the prices of the product. There is no reason it wouldn't.

Are you sure that it's not something else like the currency of your prices which are different ?
Also, do you have a price without a minimum quantity ? You can have prices with a minimum quantity, but you always need a price without a minimum quantity. Maybe that's your problem ?