I like and use Arduinos for most of my sensor applications but I've never been a fan of it for applications involving communication over a network. I have the original Ethernet shield, which was and is expensive, and I also have a version of the Ethernet Arduino, which is a slightly better value, but still pretty expensive.
The Arduino Ethernet library has evolved into a fairly useful package but, for learning, abstracts the underlying code too much to be very useful as a teaching aid.
Personally, I think the combination of a basic Arduino board, say a 3.3V Pro 328, and something like the Raspberry Pi are a much more economical approach to learning how to serve up web pages with physical data.
The recommended Raspberry Pi linux distribution comes standard with apache2 server, gcc toolchain, Python, etc. You can use that to make creating a web server as easy or as complicated as you want and get the kind of early success that's important for maintaining your interest. If you get a 3.3V Arduino, the interface is a simple 3 wire connection between the two UART's or you can use USB if you need to separate the data collection from the data serving by several feet.