I am selling my hay per large round bales out of the field, after the buyer...
There are two peices of information that will be useful. One is the custom farming rates that can be found at http://aede.osu.edu/sites/aede/files/publication_files/Ohio%20Farm%20Custom%20Rates%202012.pdf. This shows that custom harvesting of hay typically results in the farmer doing the work receiving 50% of the hay for his/her machinery and labor costs. This would leave you 50% of the value of the hay.
The second sites I would recomment are http://www.hayexchange.com/oh.php and http://www.farmanddairy.com/markets/ohio to see what hay is selling for around Ohio. Large round bales (depending if they are 4x5 or 5x6 foot bales) sell from $25 to $60 per bale. Your value should be 50% of these prices depending on type of hay and quality.
Also if you do not have any way of handling the round bales you might be a little bit at the mercy of your farmer to handle them, reducing the potential price.
I hope this helps!