Taken directly from MysticWierdo's FAQ.
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The Extras in the main menu are unlocked by making Large donations to any of the priests in the game. (Thanks to XeroFireDragonBlazer for making the post where I learned this.) You need to save the game for these to remain unlocked after a power cycle. You're able to exit without saving, load a game and save it to preserve the currently unlocked extras but to unlock more you'll need to make all previous donations again.
I've determined the required number of Large donations for most of the extras:
1 Movie: Introduction
2 Song: Beer, Beer, Beer
4 Art Gallery 1
6 Art Gallery 2
10 Movie: Bard's Tale Logo
15 Song: Bad Luck - Ogan
20 Art Gallery 3
25 Song: Bad Luck - Wolves
30 Art Gallery 4
35 Song: Bad Luck - Minion
40 Movie: InXile Logo
45 Art Gallery 5
50 Song: Bad Luck - Jail
60 Art Gallery 6
70 Art Gallery 7
80 Song: Tale of the Nukelavee
100 Song: Here's to the Bard
120 Art Gallery 8
140 Art Gallery 9
160 Song: You're a Fooling One
180 Art Gallery 10
200 Song: Bad Luck - Lava Life
220 Song: Bad Luck - Lava Death
220 Song: Bad Luck - Bard
260 Art Gallery 11
500 Movie: Ending 2
600 Movie: Ending 1
700 Movie: Ending 3
If you're going to unlock the extras, make sure you do so prior to getting on the raft in the Firbolg Mines. After this point the only accessible priest is the one wandering about the world map, who only accepts a single donation before you have to exit the map and track him down again.
NOTE: 10 Medium or 100 Small donations can be substituted for 1 Large donation. Thank to tigerbtl for making the post suggesting this possibility. If your charisma is high enough the Small donation will be free but the downside is that it would take 150 Hours instead of 1.5 Hours to unlock everything.
PC VERSION Difference: Robert Morley has reported that the extra can be unlocked faster at the kirk in West Dounby: "I've been able to determine how much larger the "Large gift" is at the West Dounby Kirk: it costs 5 times a normal "Large gift", but gives you the equivalent of 8 "Large gifts" elsewhere (i.e. 30 Large gifts gets you from Art Gallery 11 to Movie Ending 2)." This is only valid for the PC version, Xbox and PS2 player will need to suffer through the longer donation process.