The most effective way to counter an offer is to have numbers, statistics (in otherword, evidence) to support the counter. Unfortunately, what you owe on the home is not relevant.
First, you should evaluate how long you've been on the market. Is this your first and only offer? What is your monthly mortgage payment and how much is it costing you each month to stay on the market. If you haven't taken a price decrease yet, then you should also consider that. Most likely your first adjustment from $314,900 would be to $309,900. (And the next one $304,000 and the one after that...$299K)
Then, have your agent re-analyze the area comps with the most recent activity. An offer of $290k is 92% of asking. (Factoring in the closing costs, it drops to 87%) Is that the going rate for your area? Find out what the average is, and counter with that.
Have your agent find out if the buyer NEEDS the closing costs. There are good buyers out there but often they don't have the cash on hand for closing costs. If it's not a necessity, then you can counter the closing costs as well.
Ask your agent to run the various scenarios in a Net to Seller sheet to see what combination is most favorable to you. You'll see what the impact is after commissions as well.
The bottom line, you have an offer, and you should make an attempt to make it work.
Better Homes & Gardens Real Estate Metro Brokers