Go in ask to see the invoice, look at the bottom of the invoice for the "X" plan price. Tell the dealer that's the price you want. In actuality, if you get the X-plan plus $500 you're doing pretty good. But asking for X-plan is a good place to start.
If you try your best and can't get something close to X-plan no matter what. Come back on here and ask if someone will get you and X-plan number for you to use. I'm pretty sure a Ford employee can get a "friend" the X-plan price; I could be wrong.
Just note, you still have to pay taxes, DMV fees, and the dealer is allowed a $100.00 ± Doc fee above the X-plan price.
You are also entitled to any incentives currently applicable to the purchase vehicle. Ask them to run a "Smart Vincent" report to get a true listing of current incentives applicable.
I don't know a whole lot about F250's, but my read on this is, $62.5 to $63.0 is going to be about what I would expect you can get it for (not including incentives).