Depends on where you find um and their condition. A 4AGZE (supercharged 4AG that were in MR2's) can run from $500-$700. My only experience is with the 4AGE and the 4AC so I can really be of much assistance. But if I were to provide some useful information:

http://www.toyota3tc.com/

They will be able to help you more than those who frequent this forum. And if you get in a pinch:

http://www.club4ag.com/

Although most of those guys are AE86 enthusiats, many of them are knowledgeable in old Toyotas. Good luck.