If something appears too good to be true, it usually is. The ASUS ROG Ally was previously rumored to be priced at US$499.99, but according to pricing information from Twitter leaker SnoopyTech, it will actually be priced at US$599.99, which is a more realistic price point. The cheaper version of the ROG Ally will use a 6-core CPU with a more limited GPU and have half the internal storage at 256 GB, compared to the 512 GB of the "Extreme" version.
The rest of the specifications for both models are the same, including 16 GB of LPDDR5 memory, a 7-inch, 120 Hz 1080p display, and possibly the same 40 Whr battery pack. At US$70 more than the equivalent Steam Deck, it may be a difficult sell, especially since it only costs US$100 more to upgrade to the fully featured version. This is a much smaller cost increase compared to each of the Steam Deck SKUs, where you have to spend more to get more. It's possible that ASUS is using this pricing strategy to encourage most of its potential customers to buy the more expensive model.