You'll most certainly have to go used. Tablets with the features that you stated are all either Windows or Android. The good tablets with those features are all priced at or above that of the iPad, i.e., $500 and up.
There are a whole slew of cheap Android tablets out there in your price range, but you generally give up build quality, they use old versions of Android, have resistive rather than capacitive screens and are much slower. In other words, they are good for keeping on the coffee table for web browsing but that's about it. From the Amazon.com reviews from buyers, they tend to fail in a matter of months and tech support is nonexistent.
Things change when you buy a tablet made by a known/named company. Samsung, Motorola and Acer all have pretty decent reps for their Android Tablets. The HP Slate is pretty well reviewed (runs Windows 7 but is only available from their corporate sales division).
That said, one I've been looking at is the
Vizio VTab 1008. It's an 8" tablet that meets your stated requirements if you can live with HDMI out. List on it is $349, my local CostCo has it for $264. It comes with Android 2.3 OS, WiFi, bluetooth, USB ports and the microSD slot handles up to a 32GB card. And yes, it supports video calling. And unlike many non-3G tablets, it has a GPS receiver built-in. Handy for trips.
What makes the Vizio stand out is that it also contains an IR remote. You can use it as a supersmart unified remote for your home theater stuff.
The only downside I've seen so far is that some folks have complained that it is slower than the 7" Samsung tablet.
The link above takes you to Amazon.com where you can buy it for $290 with free Prime shipping.
Display: 8-inch LED-backlit LCD with multi-touch capabilities plus ambient light sensor and accelerometer (1024 x 768 pixels, native HD 720p)
Camera: Front-facing VGA camera (30 fps)
Audio: 3 speakers; SRS TruMedia enhancement
Wireless connectivity: Wireless-N Wi-Fi (802.11b/g/n), Bluetooth
Platform: V.I.A. Plus interface on Android 2.3 operating system
Pre-loaded apps: Android Marketplace, Maps, YouTube, Gmail, IM
Adobe Flash support: Yes
Processor: 1 GHz
Memory (RAM): 512 MB
Internal storage: 4 GB (2 GB user accessible)
External storage: microSD memory cards up to 32 GB
Ports: 1 HDMI (v1.4); 1 micro USB; 1 headphone jack
Battery life: Up to 10 hours
Dimensions: 6.6 x 8.1 x 0.48 inches (WxHxD)
Weight: 1.2 pounds