Having floated a few Raspberry Pi's down a river in paint cans during the removal of a dam, I can say that it is a decent set up, but struggles with resolving power in certain circumstances and eats power.

If it were me, I would look at building a whole integrated system. Start with a solid PVC tunnel and waterproofing for the electronics.

My personal approach to managing the imaging would be to create an ARDUINO not raspberry pi trigger system, because it will run much longer on less power is self restarting and very reliable if properly programmed.

You can use a motion sensor or an IR sensor or a pressure sensitive flex switch (which you tweak to get the distance and timing right to trigger the camera).

For the camera, I would look up affordable older models on the canon hack developer kit.

http://chdk.wikia.com/wiki/CHDK

You can buy craigslist canon cameras for $25 a piece and have immense control over them.

Google around and you can find a lot of different possible routes to a solution.

I knew of a professor who used a system like this to send up in a high altitude balloon. You would need a different trigger and different CHDK settings, but it gives you a starting place and I would say you could build everything including a tunnel for less than $125.....

Check it out here and google what other people are doing with both arduino's and CHDK.