Staff wages and benefits can be fairly easy to estimate. Compare the guardian program positions (whether field staff or manager) to similar programs managed by your organization.
If you know you’ll need to rely on contractors or contracted services, request quotes from potential individuals and companies so you know what fees or rates will look like.
Research the pros and cons of purchasing, leasing or renting field, safety or other equipment. Keep in mind, you may also be able to share equipment with other programs.
Things like fuel estimates can be worked out by calculating the number of trips/patrols you expect to run, distance covered, and fuel consumption/km traveled.
Remember to budget in a buffer. It’s important to anticipate the unexpected and have contingency funds available if you need them.