The SpotPass content (as I noted above, downloadable by choosing "Update Now" from your Wireless Menu's Bonus Box options) is free.
DLC... Gets kind of expensive.
Each map is sold for $2.50 to $3.00 (in the US, at least). But most of the maps (all except Apotheosis) are available in 3-map packs, which sell for $6.00 or $6.50.
In total, if you get every DLC in pack form, you're looking at $53.00 plus tax (granted, that's almost an entire triple-A game's cost, but in my mind, it's well worth it).
If you're in Europe, the packs change price, and they're being released later. If you want a full accounting (as well as a pretty decent overview of each DLC), head over to Serenes Forest. You can also change the region to European or Japanese to see what the cost is in each region (I linked you to the US page because it has a summary of all the DLCs, while Europe only has those that are released across the pond, which is about six of them).
URGENT NEWS: Fire Emblem was nearly CANCELLED before the release of this game.
This is quite shocking to me. Apparently the series had been flagging enough that an ultimatum was issued on the sales numbers. Apparently it did kick the devs enough to deliver one of the best games the series has had, but still... This is a great series that has only just been starting to be released over here in the states (I think we've only gotten five of the games in the series). I am very glad that Awakening has done well enough to surpass the numbers.
Alert your friends to this game, we need to increase sales!
EDIT: It also might be worth pointing out, this is one of the only Fire Emblem games which has had serious marketing buzz, at least in the US. The rest of the series is known and talked about by fans, but largely passed under the radar of the gaming media during the time of release. Had there been reviews and previews of the others, I'm sure they would have done better.