Forget about things like cost projection, addressable market or expansion opportunities. Those things only look good on powerpoint slides and have no real impact to the business, at least for now. At this stage, you are basically investing in people instead of the business.
It's a sad truth that the original ideas of most start up will fail at some point. Therefore, it is extremely important to find entrepreneurs who refuse to melt down along with their initial failures, and quickly find new solutions to save their products or new products/services to save their business.
I'm sure your friends will tell you that the grass is green and the girls are pretty when you ask your questions. They won't be running this business if they didn't believe so in the first place. If you evaluate your friends as incredibly tough indivituals who are comfortable with handling failures, maybe it is a good idea to invest.
Also, don't worry too much about the valuation. You don't know how much you will be able to sell your portion of the business for at least 5-8 years. If this start up can survive for that long, even inflation by itself will get you a higher price cuz your are entering at a very early stage.