Ben makes a good point, but I would add that known quality and secondary market liquidity are also important considerations. Your example, Ken Onion, spent many years establishing himself as a maker of high quality knives. Hundreds of satisfied customers can attest to the quality of his work, which provides new or prospective buyers the confidence that they are making a sound investment when they buy one of his pieces. A new or lesser known maker may offer a similar looking knife with similar materials for less money, but how does the craftsmanship and overall quality compare? The materials might be the same, but the heat treatment and construction - especially those parts hidden to the naked eye - might be entirely different.
The custom knife world has seen a few makers with questionable quality-control issues come and go over the years, and the people who bought those knives invariably lost money. If a lesser-known maker has not yet established a reputation as a consistent and reliable craftsman, then the buyer takes on more of a risk with his investment. The buyer must be compensated for assuming this risk, and that's where the lower "value" price comes into play.
Secondary market liquidity is also an important consideration for knife collectors. If a collector wishes to sell a knife in order, perhaps, to fund the purchase of a new one, he will have a much easier time doing so if the maker already has a broad customer base. This Ben Seward fellow is another great example. Ben is a full-time maker whose award-winning knives have been purchased by many dozens of serious and prominent collectors, enthusiasts, and outdoorsman throughout the country. Ben presents at shows, has an active presence on social media, is easily approachable, and has a well-earned reputation as a serious maker of the highest quality hand-forged blades. Collectors and dealers know what they're getting when they see one of his knives on the secondary market, so if you need to sell one you'll have no problems doing so.
So as a collector, you may well get a high quality knife at a lower price from a lesser-known maker, but, as with any investment, you'd be well-advised to do your homework before putting your money on the table.