I've seen at Nashbar, American classic 25.0 for about $40, but it is probably 250mm.

I've seen USE ALien posts but titanium or carbon at $130-140 but again out of your budget.

Since its an old bike, if you have the type that can transfer the whole clamp :
Pyramid Alloy Seatpost, 25.0mm. No Clamp. Length: 350mm $10.99
http://www.bikepartsusa.com/product_...ml?p=01-127871