After beating the Normal Staff Ghosts by a certain margin of time, you can unlock the
Expert Staff Ghosts. The following are the approximated times for unlocking them:

Mushroom Cup

Luigi Circuit

Normal Staff Ghost Time: 01:29.670 
Unlock Time: 01:25.742 
Moo Moo Meadows

Normal Staff Ghost Time: 01:37.856 
Unlock Time: 01.33.571 
Mushroom Gorge

Normal Staff Ghost Time: 02:16.110 
Unlock Time: 02.07.755 
Toad's Factory

Normal Staff Ghost Time: 02:22.480 
Unlock Time: 02.15.681 
Flower Cup

Mario Circuit

Normal Staff Ghost Time: 01:44.777 
Unlock Time: 01.39.183 
Coconut Mall

Normal Staff Ghost Time: 02:30.764 
Unlock Time: 02.24.788 
DK's Snowboard Cross

Normal Staff Ghost Time: 02:34.693 
Unlock Time: 02.24.725 
Wario's Gold Mine

Normal Staff Ghost Time: 02:19.583 
Unlock Time: 02.14.932 
Star Cup

Daisy Circuit

Normal Staff Ghost Time: 0:56.822 
Unlock Time: 01.48.791 
Koopa Cape

Normal Staff Ghost Time: 03:03.022 
Unlock Time: 02.54.897 
Maple Treeway

Normal Staff Ghost Time: 02:58.633 
Unlock Time: 02.50.229 
Grumble Volcano

Normal Staff Ghost Time: 02:28.237 
Unlock Time: 02.19.524 
Special Cup

Dry Dry Ruins

Normal Staff Ghost Time: 02:30.949 
Unlock Time: 02.21.681 
Moonview Highway

Normal Staff Ghost Time: 02:16.802 
Unlock Time: 02.07.016 
Bowser's Castle

Normal Staff Ghost Time: 03:04.836 
Unlock Time: 02.55.017 
Rainbow Road

Normal Staff Ghost Time: 03:05.895 
Unlock Time: 02.59.293 
Shell Cup

GCN Peach Beach

Normal Staff Ghost Time: 01:34.233 
Unlock Time: 01.30.698 
DS Yoshi Falls

Normal Staff Ghost Time: 01:16.461 
Unlock Time: 01.12.901 
SNES Ghost Valley 2

Normal Staff Ghost Time: 01:06.595 
Unlock Time: 01.03.752 
N64 Mario Raceway

Normal Staff Ghost Time: 02:14.799 
Unlock Time: 02.07.915 
Banana Cup

N64 Sherbet Land

Normal Staff Ghost Time: 02:48.651 
Unlock Time: 02.37.784 
GBA Shy Guy Beach

Normal Staff Ghost Time: 01:45.568 
Unlock Time: 01.39.255 
DS Delfino Square

Normal Staff Ghost Time: 02:41.807 
Unlock Time: 02.33.107 
GCN Waluigi Stadium

Normal Staff Ghost Time: 02:32.882 
Unlock Time: 02.24.645 
Leaf Cup

DS Desert Hills

Normal Staff Ghost Time: 02:10.233 
Unlock Time: 02.01.141 
GBA Bowser Castle 3

Normal Staff Ghost Time: 02:58.304 
Unlock Time: 02.50.637 
N64 DK's Jungle Parkway

Normal Staff Ghost Time: 02:58.264 
Unlock Time: 02.49.028 
GCN Mario Circuit

Normal Staff Ghost Time: 01:59.771 
Unlock Time: 01.55.267 
Lightning Cup

SNES Mario Circuit 3

Normal Staff Ghost Time: 01:38.880 
Unlock Time: 01.34.828 
DS Peach Gardens

Normal Staff Ghost Time: 02:34.894 
Unlock Time: 02.29.006 
GCN DK Mountain

Normal Staff Ghost Time: 02:57.744 
Unlock Time: 02.50.248 
N64 Bowser's Castle

Normal Staff Ghost Time: 03:19.323 
Unlock Time: 03.06.723