The easiest way build a base station for relaying data heard via Xbee is to use an Uno Ethernet board (it’s like an Uno but has built-in ethernet hardware, so no separate shield is needed). Then connect the Xbee to the Uno Ethernet and write a script to capture all the incoming Xbee data and send it to the appropriate web site URL. I built several systems like this over the past few years, and they work non-stop relaying several thousand transmissions each day. I’ll post a picture of the boards and the little enclosure I put them in, but it’s all relatively simple and around $100.
If your logger location has Wifi, you could also just use a Wifi-bee module and send data directly to the web from the Mayfly. Or use a GPRSbee to send it via cellular.
As for the database, if you don’t have experience with SQL and HTML, it’s not simple to set up all the database stuff and the supporting web host and pages. That’s how I did it for the past 5 years with our original system, and it’s probably too complicated for most users. But we’ve been developing a new system at http://data.envirodiy.org where users can send their data to the site where it can be stored and can easily be seen with a time series viewer. We’re working on the documentation on how to write the Arduino code for how to send data to the site, but there should be some examples in our Github repository to get you started.