I'm not sure which split you're questioning, so I'll lay it out from the start.
Say you price your song at the typical $2, or an album at $18 (very common). Of that, the Network gets 70% and the band gets 30%. While 70% may seem high, please remember that there are many people in the chain there, just as there are with traditional label distribution. That 70% gets divvied up between Harmonix (the game developer), MTV Games/Activision (the game publisher), Microsoft (XBox network for bandwidth, etc).
So, from an $18 album sale, SB looks at $5.40 in revenue ($0.60 on a single)... about on par with traditional studios, as I understand it. That 70/30 split is absolutely non-negotiable from what they say.
From there, there's the charting. You can chart the songs yourself or find someone that would do it for less than the 50% cut that RhythmAuthors.com will charge, and you can keep most or all of the $5.40/album. From what I'm told it takes about 40-60 hours of work per song to do it properly. The charting houses will negotiate -- that 50% isn't carved in stone. They'll do "up front fees only" or might negotiate a better split with a "name" band with a following that will sell better than "Joe's Garage Band" that nobody has ever heard of.
There's more than just RhythmAuthors.com out there, so you can shop around for a deal.
There's musicgameauthors.com (who does a 2/3 v. 1/3 split instead of 50/50) and interactivemusicproductions.com, whom I'm not familiar with.
To get an idea of how much the Beard could sell via this method, here's a site that lists the "unofficial" popularity of all the songs that Harmonix has directly published:
Note that these aren't exact, and may be low -- they are based on scoring leaderboards -- the number of users reporting a "solo" score on any instrument for that song. Some may buy a song but only play it "as a band", or might play without being connected to XBox Live or Playstation Network.
If I had to guess, I'd say SB would be looking at sales in the lower thousands, per song, not tens of thousands. Rock Band players tend to be younger, and download buyers often are "difficulty-driven" -- songs that are really difficult to play will sell better (hence the songs that sell 100K+ are your Iron Maiden, Avenged Sevenfold, Disturbed, etc).
If you managed a 2/3 split with musicgameauthors.com, you'd net $0.40 per song, or $3.60 per album. Sell 5,000 albums this way, and SB gets $18,000 extra revenue, with virtually no effort and ZERO up front costs. That's "found money".
And, wonderfully, there's NO cannibalization here. A sale made on RBN is not a traditional CD sale lost to the SB Store. Nobody that would have bought direct is going to say "nah, I'll just buy it in RockBand and skip the CD". The music is *locked* onto the Xbox and *cannot* be ripped to CD. What they buy can only be played in Rock Band. (And the game has to be PLAYED -- it can't just be listened to). If someone buys the music here and then wants to have it on their iPod, they have to buy the CD for traditional listening.
What is sold in the RBN store is 100% marginal revenue, and won't negatively impact traditional CD sales at all -- in fact, that $18K in revenue should lead to additional sales, both of the new CD and of back catalog.
Is $3.60 per album low compared to the net of a self-published traditional CD? Absolutely.
But with no upfront costs at all, and offering a *new* channel that does not at all cannibalize traditional sales, $3.60 beats zero all day long.