I think that 300 acres is a little small, depending on how the trails are set up.
How long would it take someone moving at a moderate pace to cover the whole property/all the trails? 45 minutes to an hour?
I worked at a 600 + acre farm, and you could ride every trail on the place in a little over 2 hours. We usually charged $15 if people wanted to haul in and ride, that's pretty standard. Riding in the State Wildlife Management Areas here is free, but there also is a private estate with 30 miles of groomed horse only trails on 1,600 acres that charges $20. To haul in. I understand it's lovely, but I usually choose the State WMAs because of the cost.
I would not haul away from home and pay to have access to 300 acres, but that's because of what I have available close by. I guess the best thing to do is to look at what's available to riders in your area, and talk to local horsepeople about what's reasonable.
ETA: Amenities that ARE important to me when hauling out to ride: Bathrooms, outhouses or portapotties. Water for the horses, or a place (like a outdoor wash rack) to hose them off after riding. Place to turn the rig around easily. Temporary corrals (not a must, but I like to let the horses roll after riding and hosing off.) Picnic tables.