What would it take to put a satellite dish 15 feet across into orbit? The weight, the volume of undeployed dish on the rocket,$$$?

You'd likely place a folding dish either on a Falcon 9 or an Atlas; the dish would weigh a few hundred pounds (and, you could get it larger, if you needed)... 

The component package is going to be the majority of the weight; The Keyhole satellites weigh up to 12 to 15 tons; Depending on what you're looking for I'd bet the instrument package would be similar. 

Keyhole satellites were huge and heavy;  they were launched on TitanIII's.  They were also designed to operate with varying conditions, have some maneuverability, and contained many kinds of telescopes and sensors... depending on the nature of your equipment, I'd bet you wouldn't need quite the amount of circuitry a KH satellite has. 

If you used a previously-used Falcon 9, you're talking about $19 million for the booster, and a probably $25 million for the boosters needed to reach orbit.  The satellite itself - from design to construction... guessing... $75 million? 

The NRO employs fleets of satellites of various types;  Keyhole is probably the one most people are familiar with (the movie "The Falcon and the Snowman" was about that), but there have been others; Corona, Gambit, Lanyard; and several others. 

 A large rocket such as the Atlas V would do it. Such large dishes have been sent all the way to Saturn.

Not much, especially if it's collapsible. 

100,000 dollars per pound for low earth orbit. Ask Elon Musk for the price, he can beat NASA like a redheaded stepchild. I suspect you want a polar orbit for your gizmos.