Some comparisons using Amazon:
Used cart prices, w/o shipping costs:
Advance Wars - $13
Superstar Saga - $10
Metroid Fusion - $14
Kirby & Amazing Mirror - $16
Warioware - $10
F-Zero - $0.25 (lol)
Golden Sun - $12
With shipping costs the general price is a little over $15. Eight bucks for a digital copy that is guaranteed to work and arrives instantly sounds fine to me. I don't see how it is expensive at all.
Edit: Oh, and by the way that's the US store. Over here in Canada land I'm looking more at $30-$60 for the titles I want, used.
I was doing a similar list with Price Charting values.
Advance Wars: $14.01 loose, $87.25 new
F-Zero Maximum Velocity: $5 loose, $17.99 new
Golden Sun: $12.05 loose, $49.99 new
Kirby & the Amazing Mirror: $13 loose, $90 new
Mario & Luigi Superstar Saga: $8.50 loose, $41.01 new
Metroid Fusion: $17.86 loose, $67.55 new
Wario Ware Inc: $9.99 loose, $35.99 new
Yoshi's Island: $11.01 loose, $36 new