Even in an era of skyrocketing prices for thoroughbreds, the figure was staggering: Conquistador Cielo was syndicated today for $36.4 million, making him the most expensive horse in history.

Breeder Seth Hancock and owner Henryk de Kwiatkowski closed the deal that will bring the Belmont Stakes winner to Claiborne Farm in Lexington, Ky., for stud duty beginning in 1983.

There had been great competition among breeders for the services of Conquistador Cielo, whom the Daily Racing Form's pedigree expert has said could become the most influential stallion in a century. Hancock had lined up buyers who were willing to pay $900,000 apiece for 30 shares in the colt.

De Kwiatkowski will retain 10 shares, which are thus valued at $9 million, even though no money changes hands for them. In addition, $10,000 is added onto the cost of each share for insurance, bringing the colt's value to $36.4 million and easily surpassing the syndication record of $30 million for the European colt Storm Bird.

De Kwiatkowski said he had received one offer as high as $60 million from a New York breeder, but he would have been forced to retire the colt immediately rather than risk defeats that might hurt his value. "It sounds nonbusinesslike," the owner said, "but if everyone withdrew the moment you proved a horse can race, racing would collapse . . . Racing would be with donkeys."

But that was one of the few sporting considerations in a hard-nosed business deal. While outsiders might think it fanciful to spend $900,000 for one-fortieth of an animal, people who understand the economics of the racing industry realize that this is an investment that is almost a sure thing.

Barry Goldstein, a New York tax adviser, explained the mathematics of it: "Typically," he said, "payments in syndications are spread out over four or five years. And, typically, the cost of one season (i.e., of breeding one mare to the stallion) is one-fourth or one-fifth of the price of a share. So you could sell your first three seasons or so to offset your payments.

"At the same time, you've got a tax shelter. You can depreciate the cost of a 3-year-old over three years, writing off 25 percent the first year, 38 percent the second year and 37 percent in the third. It's a nice game."

A syndicate member in the 50 percent tax bracket could gain a $450,000 tax saving in three years, and sell his first three seasons for perhaps $200,000 apiece, thus paying for his entire investment.

Now he still has the shares, which should have retained their value, for even if Conquistador Cielo should be a disastrous failure as a stallion, nobody would know it until at least 1988, when his first offspring will be 3-year-olds. By that time, the investors will have nailed down a solid profit. And if C.C. should become another Bold Ruler or Northern Dancer, he will become even more valuable.

There is, of course, one potential catch in this sure-fire scenario. Breeders are buying into $36 million stallions for the same reason a couple of years ago that Washingtonians were willing to pay $250,000 and 16 percent interest for modest Georgetown homes: on the assumption that the market was so strong that it would keep going up, or at least would not suffer a serious decline.

As the real estate market has shown, that can be a dangerous kind of assumption. If the bloodstock market stops booming, and seasons to Conquistador Cielo are selling for $100,000 instead of $200,000, second-guessers could look back on today's transaction and conclude that, yes, it was crazy to pay $36.4 million for a horse.