BINGO. You KNOW an agent can do a better job, but still want to undertake this important step yourself to save the money. In the long run this is dangerous, because if you are WRONG in your estimation, you could get in some trouble financially!
What if you agree to sell your house to a buyer and you underestimate the costs to you, and end up paying out of pocket?
You know the value of a good agent is not only marketing your property, but also in ADVISING you.
If you don't want to hire an agent please hire an attorney to get you through this and advise you.
Another alternative is to hire an agent on a consultative basis. You could pay an agent to provide you a good price estimation of your home's value, plus explain closing costs to you for a single flat fee. Why not pursue that option?
Not all agents will do this, however many will!
Don't go it alone. There is no one fits all closing cost estimator out there. All areas are unique in their tax rules, prorations, cloing costs, etc.
Either hire an attorney or a realtor to consult with you.