Engagement ring question?

I know this sounds weird but my guy and I have already started planning our wedding (because it is only 7 months away) but technically we are not "officially" engaged yet. Really just a technicality. But anyway, I work full time but he is in school full time so I know he doesn't have a lot of money to put into a ring. I have even said that I don't really need one (it isn't that big a deal to me) but he insists. What is a reasonable price for a ring without being too cheap? He told me he specifically doesn't want me to worry about the money, but that is hard for me to do. I want to pick out something that is reasonable but without looking cheap. So, what is a good reasonable price for a ring? I am looking at something in between 1/2carat to 1 carat.