I assume this means they'll have horrible packaging.

Nintendo should have been doing this years ago but then when you have no competition I guess you don't feel there's any need. $20 is still a little too much. In Canada they'll charge $30 and a lot of places already charge that for older GBA games. $20 would have made more sense a few years ago but now it's too little too late.
The choices are a little disappointing too. 3/4 of them are ports and I can easily find all four games new with no problems. The least they could do is re-release some of the more obscure stuff. I doubt there are many people who want those four games that don't already have them.
I think Nintendo has to realize there's a reason GBA game sales have dropped. The GBA is OLD. It's been replaced by the DS. I looked at Gamerankings the other day so see what the highest rank game of 2005 for each system was. I had the filter set to show the top 30 results. The GBA didn't even have 30 games. Some stuff is probably not reviewed enough (like That's So Raven 2 and junk like that) but GBA releases have dried up. Lowering the price of GBA games will have about as much affect as lowering SNES game prices in 1997.
I think a better idea would be for Nintendo to start re-releasing these games in collections on the DS. The Rev download service doesn't include portable games so they could include GB, GBC, and GBA titles. I don't know many people who would buy Super Mario Advance now. But a Super Mario Advance collection that includes all four on one DS cart? Yeah that would sell.