I think it depends on who you want to spruce up your collection for, yourself or to show off to people. I'm going to assume here that you want to show off your new card to friends and family who are not familiar with baseball cards that much.
$100 : a decent Mantle
one example:
http://cgi.ebay.com/1958-Topps-487-M...-/110520353348
$200: Reggie Jackson rookie card
http://cgi.ebay.com/1969-TOPPS-RC-RE...-/120497332866
$500: Ty Cobb T206
http://cgi.ebay.com/1909-1911-T206-T...-/130382805558
$1000: a nice Lou Gehrig or lower quality Ruth
http://bid.robertedwardauctions.com/...x?itemid=13639
http://bid.robertedwardauctions.com/...x?itemid=13610
$5000 and up: T206 Honus Wagner
http://bid.robertedwardauctions.com/...x?itemid=13090