A few years ago I was seeing an American girl, I was planning to stay for 60 days. When I told the guy at the gate I was coming to see my girlfriend they asked me about how I was funding a long trip. I was a student and I told them honestly I had < 1000 in my current account but I had a credit card with no balance and about 2000 balance available to spend. I might even have told them that the next instalment of my student loan was going in before I left the country (also true).
I told them I was staying with her and they waived me on my way. I didn't get the impression they were after a fixed amount, I think it is a judgement if you seem plausible in your explanation. They didn't even ask me to prove I had the funds that I claimed. My relationship lasted two years and I entered 4 times over the course of it. This was the only occasion they showed any interest in my finances.
Your girlfriend should carry the evidence and make sure her answers tally up if they do ask.