“White” gold is not a variant of elemental gold. It is an alloy, a metallic mixture—gold with other metal or metals that result in a white appearance. Thus, you could never have 24 K (pure) white gold. White gold is often 12 karat, which means it is only about 50% gold.
White gold should not be significantly more expensive than yellow gold of the same karat designation. If a jeweler is telling you that, he’s messing with you.
White gold is generally more expensive than sterling silver but, unlike sterling silver, does not have a set and internationally standardized metal content. Sterling silver is silver mixed with copper and should have only trace amounts of other substances. White gold can have god-knows-what mixed in with it, including nickel which can cause itching, burning, and skin rash in sensitive people.
As sterling silver is inexpensive and less toxic than anything containing nickel, I always prefer it to white gold, except in windy coastal environments. In a few days, salt vapour from sea air can tarnish sterling silver more than months or years in the air of New Mexico would do.
A white gold alloy consisting of gold and silver is called electrum. It was valued by the ancient Egyptians more than purer yellow gold, because it tarnished less than pure silver, but was whiter and rarer, in Egypt, than yellow gold. Electrum does not resist corrosion so well as higher-percentage gold alloys, but it survives much better than pure silver and silver-copper alloys. Several electrum items are among the famous treasures of King Tut’s tomb. Unfortunately, electrum is not a commonly used term in the jewelry industry, so you cannot enter a typical store and confidently pick out a nickel and rhodium-free electrum ring or bracelet.
A good artisan jeweler, however, should be able to make electrum objects for you. Gold and silver alloys are easy to create. Harder electrum-like alloys containing a bit of copper as well are not so easy, but not difficult to concoct. Electrum without copper is flexible, soft, and easy to work. It is not good for setting very valuable stones in inconspicuous delicate prongs, as the prongs may deform with impact and allow the stone to slip; but it is very good for full bezels, and for any fairly bold, compact, or chunky piece of jewelry.