Depends. My rule has been: I will put you, the owner, in touch with the property's past. You get ALL antiques and relics such as old tools, antique toys, etc. I tell them that although the mineralized soils tend to eat the metal on coins and generally they come up worthless to coin collectors, I like finding old coins just for the old dates(which I do). I also show them examples of relics and/or toys found and given to landowners. Use your cellphone to store pics of those finds that are cool nut not on YOUR target list and show it to them. Often that is a positive trigger for them to allow you to hunt.
I think Richie has some good thoughts. For me, it totally depends on the type of property and the historical significance of the property. If I had the chance to detect some place like the local Marcus Daly Mansion here in Hamilton Montana, I would donate EVERYTHING to the mansion to be put on display, and might even get an "archie" (heaven forbid!
) to document the finds and their history / location on the property.
The "in between" would be something like hunting a historic house, Civil War battlefield on private property, or the like. Work out something agreeable to you and the property owner in advance.
At the other end of the spectrum, if it is just a typical 1930's home front yard, then if the homeowner wants to invest $500-$1500 for a decent metal detector, spend a year or two learning how to really use it well, then sure, they can recover and keep the 3 silver dimes worth $5, the Buffalo Nickel worth 10 cents, and some wheat pennies and modern coins in the yard. When asking permission, if a homeowner seriously asks "What's in this for me?" I politely thank them for their time, and move on. Most folks could care less about what is in their yard. They give the ok, and then go back to watching TV or head out to run errands. I do share some of my metal detecting finds with the homeowner IF they are interested, and the coins and relics are of MY choosing. The homeowner would go to their grave never having the motivation or interest in coins or relics until I came along to ask, so I feel that the choice of items to keep are mine for doing the research, having the skill and expertise to locate said items, and having the equipment to safely recover the items.
Of course, detecting is a ton of fun no matter what type we do, but don't sell yourself short in terms of having some great items for YOU to keep for your hard work and efforts! Good luck with your hunt!
I can send you my address if you like. 
