This is a dilemma indeed. I will try to answer your question to the best of my ability. I would like to ask you a clarification question (or rather have you ask that to yourself). 1. Are you looking to make machines that work in space? 2. Or are you looking to dream about understanding the workings of the cosmos by trying to theorize about black holes and dwarf stars and whatnot and/or finding new habitable planets by using wonderfully high end telescopes?
If you are thinking of designing, creating, and making machines that will help assist space exploration expeditions, then Mechanical Engineering/Aerospace Engineering would be a way to go. If your answer is more along the lines of stuff in the second question, then astrophysics (usually a PhD) is what you need.
Many people are misunderstood by the disciplines in engineering. You don't have to have a degree in Aerospace Engineering or Space Engineering for Astrophysics work. As you said, Mechanical does give you a wonderful perspective into many other engineering disciplines and it can help you "create" a path that is important to you rather than you taking a predefined path which may or may not completely satisfy you. (I would like to clarify by saying that I am a bit biased towards Mechanical Engineering because I am a Mechanical Engineer and I have seen many with an ME into Aerospace, Biomedical, Biology, Chemistry, Industrial Engineering, Electrical, and many more).
Now as of your main issue regarding the personal and monetary investment, although it is true that getting another bachelors would help you reach your goal of doing something in the space industry, I would recommend sticking with economics as a profession (given that you still like it) and learning engineering through online videos, lectures, books, etc. Besides, remember that you can always take like 1 class per semester for fun and keep learning science and engineering on the side. Most of the people will tell you that engineering is hard and very time consuming and it is almost like having a full-time job. This is true for most, however, if you are truly passionate, you will be able to juggle economics and engineering. And by not committing yourself completely to the engineering field, you are keeping the option of leaving engineering always open. Practically speaking, I think it makes more sense to continue Economics but for the sake of sanity, do not lose hopes and do not leave Engineering. It is a beautiful discipline.
Lastly, I don't think that you should feel too bad about your GPA. In most cases, grad schools look at your GRE first, then your research that you may have done at your undergrad institute, and then your GPA (I am quoting one of the graduate studies advisor that I met at a grad school fair). If you have had some experience doing research, then it is a really good resource. If not, try to get as good of a score in the GRE as you can. I would say that you should think about this from the most practical perspective as you are trying to build a career for yourself and making a hasty decision without putting much thought into it may not help.
If you want more time to think, get a job/internship at an economics firm and see if you still like what you would do there. Then go get an internship in an engineering firm (I know that this will be hard for you given your background but you can always ask a company to let you volunteer without getting paid). Shadow an engineer, learn a thing or two about the engineering industry from them. You will then know where your heart and soul belongs.
Check out this professor at San Jose State University. See how he fulfilled his passion for science after getting a PhD in Economics. I highly recommend asking him for some advice also. http://www.sjsu.edu/faculty/watkins/resume2.htm