As an 11 year O-4, I find it pretty easy to make about $2,000 per month by working about 7-8 days.

Depending on your unit and aircraft, there's also other opportunities. I'm a C-17 guy and I could volunteer for a 12 day mission that typically pays about $3,500 + per diem + tax free/hostile fire. Other units will put guys on longer term orders (45-120 days at a time) and essentially you'd make active duty pay, benefits, and per diem during that time.

As a traditional reserve "bum" (no civilian job) in a mobility flying squadron, it's reasonable to make anywhere from $4,000/month all the way up to full active duty pay.