This is the big one, 300 songs... it's kind of a lot. All that money...
Let it be known though that you can get very close to this using some disc exports, or... even past this if you get almost all of them. The disc library is as follows:
Rock Band - 55 songs.
Rock Band 2 - 79 songs.
LEGO Rock Band - 45 songs.
Green Day: Rock Band - 47 songs.
Rock Band: AC/DC LIVE - 18 songs.
Rock Band Track Pack Vol. 2 - 20 songs.
Rock Band Track Pack: Classic Rock - 20 songs.
Rock Band Track Pack: Country - 21 songs.
Rock Band Track Pack: Country 2 - 21 songs.
Rock Band Track Pack: Metal - 20 songs.
This rounds up to 346 songs. More than enough, right? However, you may not want to buy all these disc packs because there might be some songs you may not personally like, that's understandable. So how about this?
New copies of Rock Band 2 contain a code that allows you to get 20 songs for free.
There's also free DLC you can get from the store! It's not much, but it should help.
?Am I Crazy - Little Fish
?Charlene (I'm Right Behind You) - Stephen and the Colberts
?Dreaming of Love - Lights Resolve
?Headphones On - Miranda Cosgrove
?Promised Land - Vesuvius
?RBN Megamix 01 - Various Artists
?Still Alive - GLaDOS
Alright, so here's another breakdown. Using just the original games Rock Band/Rock Band 2, you'll get 134 tracks between them.
Including the free pack with Rock Band 2, and the free DLC listed above, we now have 161 tracks. Rock Band Blitz tracks count towards this, making our total now 186 tracks. Let's throw in LEGO Rock Band now, you may not think all the songs are great, but consider that they will help towards more than just this achievement. Adding this now gives us 231 tracks in total.
Now, because we're also going to work on "Loyal Fan," we're going to need at least ten songs by the same artist/band, which in turn will help us towards 300. We should have at least one song by one of the artists that will help count towards this, so let's round this at 240 songs.
So we're still short here, and you may not want to pick up every track pack to help out, so let's check out the Rock Band Network content.
You could pick up 60 songs which are being sold for 80MSP, which in theory will set you back $60, €50 and so on. The disc packs will probably be cheaper in the long run, and they have achievements included... if that detail will get you interested.
Needless to say, after giving a rundown here, this will be a pretty expensive achievement, don't rush for it if you're low on songs, get what you want to play so you at least get some enjoyment out of it. The game should be fun, not an investment!