WHEN it comes to commercial cake mixes, the proof, decidedly, is not in the pudding.

In a recent tasting of six common commercial (white) cake mixes, the uniformity was surprising. There was very little difference among name brand, store brand and even those pudding-included mixes. The only marked difference was price, which ranged from 50 cents to $1.09 for two layers of white cake.

Washington White Cake Mix is priced much lower than the others and can be purchased at Safeway for 25 cents per box (one-layer yield). Thus two layers (three cups of cake batter) cost 50 cents, compared to about 85 cents for two layers of a store-brand cake. National brands hover around the $1.10 mark. Both store brands and national brands yield four cups of batter.

White does not, in this case, mean vanilla. None of the cakes claims to be vanilla, and one taste tells why. The most flavorful cakes taste strongly of vanillin, an artifical vanilla flavor that is sometimes more reminiscent of artificial coconut, but for the most part they taste nothing but sweet, in a spongy commercial-white-bread sort of way.

Tasters (all admitted scratch-cake fans)) learned that they would rather have had their cake than eat it. The cakes were uniformly browned and differed only slightly in depth of browning and layer appearance. This one attribute, however, would be, in most cases, quickly smeared with frosting. In this tasting, the sweetness of the cakes did a remarkable job of overwhelming the flavor of homemade chocolate frostings. The even crumb of the layers was too moist, in some cases gooey.

Add preparation cost to the initial price of the mix, and you get an even wider price differential. The more expensive cakes, ironically, call for more ingredients, making them that much more expensive. Cheaper brands and store brands require two egg whites; the name brand mixes often require three egg whites, and one calls for additional oil (which increases the number of calories).

The bottom line is that the best value in cake is that which is the cheapest--at least among the brands tested. The Washington White Cake Mix contains a little less actual mix than the others, so it will yield a little less cake, which means a thinner layer, but one that does not differ noticeably in quality.