Depending upon where you live in Michigan, this may not be the time to sell your home. Will you be purchasing a larger or more expensive home? Will you be moving to an area where home values are rapidly increasing? Are you in danger of loosing your home to foreclosure? If the answer to these questions is No, then I would really think hard about selling in this market.
If the answer is yes to any of these questions, then David is absolutely correct. Pay the 300 to 500 dollars and get a "not for refinance" appraisal for your home. I know this is not what you want to hear, but it is what you need to hear in this market.
Any of the agents from Michigan below would gladly give you an honest opinion of the value of your home. Unfortunately, I doubt that you would pay attention to their advice.
The best bet is to have an agent (or 2) prepare a Comparative Market Analysis. The report will show you the recent sales prices of home similar to yours, in your same market. I am assuming you are talking about your assessed value from the township/city. Those are calculated with generic figures, not specific information pertaining to your home. A CMA will make adjustments and take into account your home's features. Your home is really worth what a buyer is willing to pay - and that can be estimated by the amount other buyers have paid for the similar homes. Good luck! Please feel free to contact me with any specific questions.
If your "valuation" was not done from an actively listing agent then google Realtors in your city and meet with 3 of them and see. If they all come within 5% of each other then I would think it is accurate.