Some of your requirements may be difficult to meet.
A combined temperature and humidity probe that has a very small footprint, is protected from excess moisture, and has a long (10ft+) cable from the probe itself to the controller
Why not put the controller nearer to the probe? Long cables require additional processing/hardware.
- Controller displayed readings of temperature and humidity based on probe number
How many probes do you figure?
- Computer displayed readings of temperature and humidity based on probe number as well as the ability to log the data and make calculations from it (e.x. 24 hour high and low temp/RH, daily averages, weekly averages, etc)
Using the logged data is something that a spreadsheet on a PC is better suited for.
- The ability to turn on and off a device (e.x. a fan) based on probe readings
One device per probe? Multiple devices per probe? Multiple probes per controller? Soon, it gets to be a nightmare configuring what to do when for what probe.
The only comparable system out there is the HerpKeeper (from digitalaquatics )which is way overcomplicated and expensive in my opinion.
What price point for the device do you have in mind? How is the data to be logged? SD card per controller? XBee on the controller sending data to a central location? Via ethernet on a server somewhere?
and have spent thousands and thousands of dollars on their frogs.
But a cheap system of monitoring the environment is needed?