There seems to be a bug in WPAdverts which does not allow to use the price field without decimal places to fix this you would need to open file wpadverts.php find there code
wp_localize_script( 'adverts-auto-numeric', 'adverts_currency', array(
"aSign" => $currency["sign"],
"pSign" => $currency["sign_type"],
"aSep" => $currency["char_thousand"],
"aDec" => $currency["char_decimal"]
));
and replace it with
wp_localize_script( 'adverts-auto-numeric', 'adverts_currency', array(
"aSign" => $currency["sign"],
"pSign" => $currency["sign_type"],
"aSep" => $currency["char_thousand"],
"aDec" => $currency["char_decimal"],
"mDec" => $currency["decimals"]
));
The same update will be in next WPAdverts release.