Its quite late, and I'm rushing this, but I was thinking something along the lines of:
They are purchased using energy charges. (10 for the first one, 20 for the next, etc)
You get them twice a day (daily reset, twelve hours after).
When you use them, you get 1 energy block per scout you have.
One energy block can be sold to effectively get 5 energy.
So you'd be putting a lot of energy into these scouts but they'd eventually be worth it. The fact of using energy to purchase them eliminates the divide, if it were a cash thing, the richest players would get them first. This way, everyone can use their own energy to do so, but the market adds that element of variation and whatnot. Eh. Just a suggestion.