Step-through Sports go for a LOT less than the diamond-frame versions.
I've bought a half-dozen step-through "lady" (I hate that term) Sports for $40 or less in really good mechanical shape with very low miles. They needed to be fully tore down, cleaned, polished, all the bearings repacked, and then set up again from scratch because all the grease had dried up in the last 40 years and attempting to ride them without doing this first would destroy them.
Then I turned around and sold them for around $200-300. Time was a couple of years ago when the "Tweed thing" was at full steam I could sell a really nice example for as much as $350 after buying it for $40 and investing about $50-75 into parts (mostly tires, and brake cables/pads.) Some of these bikes looked in showroom shape when I was done. Many of the "lady" sports had so little miles on them that the mold nubs were still on the original rotting tires.
I still see these bikes showing up on the Chicago Craigslist site for under $50. $100 is about the max I'd pay for one and only if it had new tires/brakes already. It depends on how hard they are to find in your local area and how much of a hurry you are to acquire it.
I suppose if you want to buy one that someone else has already fixed up and put the money into it'd be a lot more. But why? Most people mess these bikes up not knowing what they are doing. I'd rather find one that fresh out of the barn and is NOT pre-mangled by some hamfist hack who thinks a hammer is a proper cottered-crank tool.