@ Umbran: One wouldn't have to feed hippgriffs dwarves in a D&D world. Create Food type spells could be used to feed the hippogriffs, if the GM desires a hippogriff cavalry in the world. In this case, the cost of housing the necessary clerics is less than that of the feed stock previously mentioned.
However, even in the real world, not all creatures reproduce well in captivity. If the GM should choose to disallow a hippogriff cavalry, there is a logical basis for his as well. Indeed, the high price of hippogriff eggs is a logical extention of their inability to reproduce well in captivity -- if you could farm them like chickens, they'd be "cheep"er.
RC
You could expand on the ideas and solve (and later on raise) most of the included problems. Create a magical feeding system for the hippogriffs that´ll remove the need of clerics. Use pregnancy charms. Use a hippogriff rutting spell, and so on. In a magical world, all of that would be possible. And quite cheap, too.