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
Tim Berry is the chairman of Eugene, Ore.-Palo Alto Software, which produces business-planning software. He founded Bplans.com and wrote The Plan-As-You-Go Business Plan, published by Entrepreneur Press. Berry is also a co-founder of HavePresence.com, a leader in a local angel-investment group and a judge of international business-plan competitions.