When Design Miami opens on December 3, a new exhibitor will be among the 25 participants. The Matthew Marks Gallery, best known for representing top-tier artists such as Andreas Gursky and Ellsworth Kelly—and a fixture at Art Basel Miami Beach, where it will also have a booth—is bringing to the design fair a selection of whimsical tables and chairs by the Seattle-based artist and wood-furniture maker Roy McMakin.

Marks’s participation has provoked a range of reactions from the design community. Some are ready to welcome the dealer, particularly bringing work like McMakin’s. “I think the design fair is a good venue for it,” says Suzanne Demisch, of the Demisch Danant gallery, in New York.

But others fear this foreshadows future competition with deep-pocketed contemporary dealers who, attuned to the growing collector interest in modern design, are eager for a slice of the pie. “There is concern from some of the old guard— the established furniture dealers—that they may be pushed out by the art galleries,” says the New York design dealer Cristina Grajales.

“It’s good and bad,” says Zesty Meyers, a codirector of the R 20th Century gallery, in New York. “It’s good because it brings more money into the market, but on our end, we have to push to keep our position.” While design dealers may benefit from a widening group of collectors, they may be at a financial disadvantage, particularly when it comes to building their stock. Prices for the category have escalated in tandem with those in the contemporaryart market. Some blame relentless media coverage of a few superexpensive trophies, like the Carlo Mollino table that sold for $3.8 million at Christie’s in 2005, for the design inflation and the heightened sense of competition at design fairs. Whatever the reasons, the average lot value at Sotheby’s sales in the category has more than doubled in five years from $30,000 in 2003 to $80,000 today. Such sums don’t mean much to a Russian billionaire collector like Roman Abramovich, who was spotted shopping at last June’s design fair in Basel, but for cash-conscious dealers trying to beef up their inventories, they can be a shock.

“If you were buying pieces for $5,000 five years ago, you are vying for a $50,000 piece today,” says Meyers. “How many can you afford to buy a month? On top of paying the bills, it becomes drastic.”

Having prestigious contemporary-art dealers invested in design “is good news for the value of the property,” says James Zemaitis, Sotheby’s director of 20th-century design, “but terrible news for a culture of design dealers who are not capitalized to compete.”

And the competition is over not just design buyers and objects but also the creators. Design artists have already begun gravitating to art galleries. Powerhouse Gagosian represents Marc Newson, while the London art dealer Timothy Taylor recently signed Ron Arad. Further defections are sure to follow. “It’s certainly going to happen with the stars,” says Grajales. One motivation is obvious: By exhibiting among more-expensive art, designers can expect heftier price tags for their own pieces. Although contemporary artworks routinely sell for more than $1 million at auction, Newson is the lone designer to have crossed this threshold, with his sleek metal 1986 Lockheed Lounge, which fetched £748,500 ($1.5 million) at Christie’s London last October. Meyers estimates annual sales for the postwar design market worldwide at about $300 million, versus the contemporary art market’s billions.

Design may never reach those stratospheric heights, but the profit potential of category crossover has reassured some dealers. “It has forced us to pay more but allowed us to charge more,” says the New York dealer Paul Donzella, who specializes in 20th-century furniture. Demisch is similarly sanguine. “It’s about being viewed on the same plane as art dealers, under the same umbrella,” she says. Indeed, certain design dealers are partnering with contemporary galleries to provide a new context and clientele for their wares. R 20th Century teamed up with the Sean Kelly Gallery, in New York, for a show of the Danish designer Poul Kjaerholm’s furniture. And this month, the London dealer Kenny Schachter presents the architect Zaha Hadid’s sculptural objects at Sonnabend in New York.