You need to know two things to set a price.
a)Cost plus - what it will cost you plus a margin. Note: as you have already written the software, that cost is $0 - you need to look at costs directly associated with the sale.
b) Value Add - what is it worth to the client. How much will he pay for it and be happy with the purchase.
If a) is more than b), don't do business.
If a) is less than b) - negotiate.
For your software and situation, I can't help with b), but with a) maybe some insight:
I expect from the post that the software was written by you, for you, for a specific job. If so, the cost to sell it to one person is a *5 - *10 multiplier (A magic number taught to me at the school of life) - if it took 10 hours, it will need another 40 - 100 to make it usable for the customer.
Then there is the cost of sales, post sales and post sales support. You cost of sales is low (he phoned you). Post sales support is not technical support - it's the stuff you to keeping in happy, discussing the next new feature of bug fix etc. It is not the fixing or improving his software. Then there's the cost associated with supporting the software when he wants changes.
All the above need to be worked out before you can set a price.
The other way is to give it to him, you won't make any money, but you won't loose any either.