When you approach an end user and solicit them to buy a domain, it's etiquette and common sense to have a price in mind to tell them, if they show interest;
However, since they initiated contact with you, the ball's in their court if you want it to be. A common and honest tactic can be to say that you are using this domain and hadn't really considered a selling price, and to please make an offer in the budget they were considering.
In the meantime, decide on what you think is a realistic price that would make you happy, below which you would not sell the name. Take into it all factors, like any traffic and income you make from the name, other possible buyers, exact search numbers for both 'miami oh' and 'miami ohio', other extensions taken (miamiohio is taken in a few ext, yours is only in dot.com), and other combinations that this buyer might buy at reg fee if your price is too high (interestingly, miami-oh is taken in com/net, while miami-ohio, which gets many more searches, is available to reg). They can also reg MiamiOhU.com or MiamiOhioU.com or UofMiamiOh.com... which are a little longer, but if it's a savings of many thousands of dollars, they might do something like that instead, since it's just another letter or 2
Personally, for your domain I'd ask them to make an offer.
If you would rather set a price yourself, I'd probably ask $3500 but settle for 2500 if they counter offer.
That's just me. I think that's the 'sweet spot' area, for this size of a university. Ask much more than that and I think they'd look at other possibilities.
But who knows? Maybe they'd pay much more... or maybe they were only thinking of $500, you never can tell until you open negotiations.
GL, and congrats on having an end user approach you for a buy
