How expendable were the Nazgul to Sauron

The answer is that your question is flawed, at least in part. Sauron did send a Nazgul. As Grishnakh and the other Orcs are bickering, one of them says:

'I came across,' said the evil voice. 'A winged Nazgul awaits us northward on the east-bank.'
-The Lord of the Rings, The Two Towers, Book III, Chapter 3: "The Uruk-Hai"

The "evil voice" is Grishnakh, of course. So Sauron did indeed send a Nazgul along, possibly the one who had his mount shot out from under him, but probably not. Shortly after the passage quoted above, Grishnakh runs off from the main party of Orcs and then returns a bit later.

'Splendid!' laughed Ugluk. 'But unless you've got some guts for fighting, you've taken the wrong way. Lugburz was your road. The Whiteskins are coming. What's happened to your precious Nazgul? Has he had another mount shot under him? Now, if you'd brought him along, that might have been useful - if these Nazgul are all they make out.'

'Nazgul, Nazgul,' said Grishnakh, shivering and licking his lips, as if the word had a foul taste that he savoured painfully. 'You speak of what is deep beyond the reach of your muddy dreams, Ugluk,' he said. 'Nazgul! Ah! All that they make out! One day you'll wish that you had not said that. Ape!' he snarled fiercely. 'You ought to know that they're the apple of the Great Eye. But the winged Nazgul: not yet, not yet. He won't let them show themselves across the Great River yet, not too soon. They're for the War - and other purposes.'
-The Lord of the Rings, The Two Towers, Book III, Chapter 3: "The Uruk-Hai"

Sauron sent the winged Nazgul, but ordered them not to cross the River Anduin, probably because the first winged Nazgul sent out was immediately shot down by Legolas. Since Ugluk tauntingly asks Grishnakh if the Nazgul has had "another mount" shot under him, we can probably assume that they already knew about the first mount being killed, and therefore, the Nazgul who "awaits" the Orcs on the eastern bank of the river is a different one.

The Middle-earth Encyclopedia entry on Grishnakh says:

Grishnakh and Ugluk had an ugly dispute in which Ugluk cut off the heads of two Mordor Orcs who opposed him. Grishnakh reported these events to one of the Nazgul on February 27, but the Nazgul advised Grishnakh to pursue the Uruk-Hai and remain in the company with the Hobbits. On February 29, Grishnakh and his band of Orcs from Mordor once again joined the company of the Uruk-Hai. The Orcs and Uruk-Hai rested near Fangorn Forest.

And Thain's Book contains the following passage in the entry devoted to Nazgul:

Sauron kept the Winged Nazgul east of the Anduin at first. However, on January 9, 3019, an unexplained shadow passed over the Fellowship in Eregion (FotR, p. 299). Some have speculated that this was one of the Winged Nazgul that crossed the river prematurely (HoME VII, p. 365).

According to a timeline published in The Lord of the Rings: A Reader's Companion, one of the Nazgul met with Grishnakh on the eastern side of the Anduin near Sarn Gebir on January 26. Grishnakh was an Orc from Mordor who had information on the Fellowship's journey.

Grishnakh and a company of Orcs along with a Nazgul attacked the Fellowship on February 23 as they were travelling downriver by boat. Legolas shot and killed the Nazgul's steed with an arrow, but the Nazgul survived. Grishnakh later joined forces with Ugluk of Isengard to attack the Fellowship at Amon Hen and Merry Brandybuck and Pippin Took were captured.

Grishnakh met again with one of the Nazgul on February 27. The Nazgul told Grishnakh to pursue Ugluk, who was taking the Hobbits to Isengard. But Grishnakh was killed by the Rohirrim and Merry and Pippin escaped on February 29. That same day, Frodo Baggins and Sam Gamgee heard the cry of a Winged Nazgul overhead while in the Emyn Muil. It is possible that this was the Nazgul who had been Grishnakh's contact.

As the previous answer suggests, Sauron deemed the fell beasts too valuable to risk being lost before the war began in earnest. He sent one to oversee the capture of Merry and Pippin, but made them hang back a bit to stay out of danger.

We can probably assume that Sauron deemed the actual capture of the hobbits to be too dangerous to get the fell beasts involved, because Legolas had shot down the first fell beast that was sent out, and the Orcs were carrying the hobbits through hostile territory with the Three Hunters and the Rohirrim in hot pursuit; therefore, he sent Grishnakh to catch the hobbits and bring them to the winged Nazgul, who would be waiting in a safe place, and would then fly back to Mordor with the captives.

We might think of Grishnakh as a sort of Major or Captain, and the winged Nazgul as a Lieutenant General. The General is in command, but too valuable to send to the front lines, so he stays away from the battlefield and gives orders his lower ranking officers who are expendable and in the thick of the action.


Note: the quotes from Thain's Book and Middle-earth Encyclopedia are based on the same source, The Lord of the Rings: A Reader's Companion which the invaluable Tolkien Gateway describes as "one of the greatest recent secondary works on Tolkien's world."