Really depends on how many bales and how far. 10 loads @ 10 miles round trip means $50 is far too cheap. Not cheap to keep things maintained, especially anything with tires. I think a dollar per bale, multiplied by number of miles one way would be reasonable to determine a price per load. So 8 bales at 5 miles one way would be $40 a load.
In the end, if the neighbor thinks you overpaid, let him decide what it was worth. Far better to be generous with a good neighbor than to try and do it as cheaply as possible.
Anything goes wrong (blown tire, etc.) you replace and deduct from what you owe him before you return trailer.
Your fortunate, I would never let anyone else use my equipment, but I would haul it for you.