iSuppli have reported on their teardown of the Nexus 7 - $152 BOM, if you hadn't seen it already.
Interesting to see how the Kindle Fire BOM has dropped from $191 to $139.80, in the 10 months since it launched. I'm not from a hardware background, but do manufacturers generally launch with the intention of making low margin on early adopters and compensating for that with mass market take-up once prices are lower? If so, given the buzz around decent low-cost tablets, one could consider this low initial margin to be advertising.
How long does a device like this continue to sell for - a year or more? If the Nexus 7 takes the same path, then even sales of the low-end version should be generating a reasonable profit in 6 months time, or leave room to drop the price further if Apple come in low with a rumoured/FUDding (delete as appropriate) smaller iPad.
I also wonder what the "additional costs" iSuppli talk about would include - shipping, packaging (or would the latter be "box contents")?