- With camera if you have some high vantage point which sees the whole field, it may help reduce the cost by needing only a single camera, if you can scan and zoom in the area. Maybe infrared/thermal camera is also helpful if the attacks happen at night. The human infrared signature is easily identifiable at night : a human sized blob of the right temperature.

- If you don't have a high vantage point, you can try using a balloon anchored via a rope (hydrogen (cheap but illegal) or helium).

- (If you like the technological challenge) Quadcopter drone with ardupilot at random intervals : goes up, snap a picture, goes down and land and wait on the ground to preserve battery. You can also have it land on a base station to recharge automatically. Probably hard to do when weather is not nice.

- Maybe there are some unused at night road to access the area that are likely to be used by the aggressors. Either positioning a camera there, or car counting device / induction loop may be a good bet.

If there are multiple access point, you probably can incentivize the intruders to use the easy way-in but discreetly controlled by for example putting barb-wires, electric fence everywhere, warning signs, fake camera boxes, (airsoft motion activated landmines ?), cheap radar motion detector coupled to sound alarm, ...

Usually if there are significant visible signs that the property is well monitored, intruders will pick another easier farm.