Oh for fvcks sake, you're actually worrying about whether or not to split the cost of a DNA test? If you want one, then just pay for it. Insisting on her paying half will only further stir the pot of bad will between the two of you.
That being said, I think you need to consider what you'd do with the results if you do get a DNA test. What if you want to stay with your girlfriend and raise this child as your own? If you do, then don't worry about a DNA test.
However, if you've broken up and don't want future contact with her or the child if it's not yours, then get a DNA test.
I don't think that the future of this relationship should hinge on the lottery of who's sperm fertilised the egg.
Never regret anything that has happened in your life. It cannot be changed, forgotten or undone. So, take it as a lesson learned and move on.