There are professionals who can help you. The problem is that valuations can go all over the map, depending on which formulas are used.
If you have a small business attorney you trust, or an accountant you can trust, they might be good resources. Also, consider taking the problem to one of the more experienced people at the nearest Small Business Development Center. You can find out more about them here.
Related: How to Value Your Startup
Related: Investment ROI Calculator