Field engineers start out somewhere around $50k/year with a $10k sign-on bonus on top of that. Once you've been here for 9 - 12 months, you should be able to give what we call a breakout presentation to your peers and managers. If you pass that, you become eligible for field bonus. For our area, it's $150 / day up to $2500 / month. We also offer per diem to cover your meals that you take to the field. That can be a few hundred dollars a month depending on how often you are out there.
I am in the office quite a bit more than some of the other guys as I'm transitioning to something different and have different responsibilities. I'd say I'm in the field 65% of the time and do not max out at that $2500 / month often. I brought in just under $97k last year, i.e. 2nd year in the company.
Keep in mind this is also on top of insurance, 401k matching, employee stock purchase plans, etc. You will make more with the smaller guys, but they don't always offer all the extras. Also, notice I was hired in May '09. Pretty bad economy around that time, so I also like the stability here. Work your ass off and you're going to have a job.