Radical suggestion: don't make the ring a surprise. Make the proposal a surprise, but not the ring. Bring your girlfriend along as you do research and have her give you ideas on the setting/size/gem type. (If you happen to find similar items for cheaper, she doesn't need to know! For example, if she is set on a diamond, there are "qualities" that they measure that are invisible, like fluorescence. Ones with higher fluorescence are cheaper, even though they look identical except under ultraviolet light. She doesn't need to see the exact stone specs! I have no idea what the specs are of my diamond except that it is highly fluorescent because my DH proudly told me he got such a great deal on an amazing stone precisely because of this "defect").
You can tell her: "I want you to be happy when you look at this ring and feel how much I value and love you. So I want you to get what you want. With that said, the less we spend on it, that leaves a bit more leftover to spend on a great wedding party (or something else she would value). Some ideas I have to keep the cost down would be a lab-grown diamond -- I can get you something higher quality and its more ethical that way! Or I know your favorite color is red. I could get you a gorgeous ruby and your ring would be really unique and personal. What do you think?"
The trick is to "pitch" your ideas in a way that appeals to her. If she isn't as focused on saving money, then you telling her this will save money may not go over well as she might read it as you don't value her. If instead you show her how these slight changes could benefit her in ways she likes, she would be more likely to eagerly get on board. And both of you get to be happy that way!
My engagement ring was about $1000 USD. My wedding ring, about $150 (it is a plain silver band). My DH's wedding ring we splurged on, that was about $500.