I read more thoroughly the old thread. Wanderer and Mánagarmr had some very interesting Q&A.
It appears there is a common ground and some balance space. The reasonable common ground is immobile, cloaked scout built by a mobile builder. The balance space concerns cap, energy cost, knowledge cost and need of supply or not.
On the question of supply, the difference is frontier-only or any neutered planet. As the main goal is to keep an eye on CPA, hybrids, threat and the like, I personally think the required supply is a good limitation that would allow a low price.
For the record:
Scout MkI: 0 Knowledge, 10 cap, 25 energy.
Scout MkII: 500 Knowledge, 20 cap, 250 energy.
Scout MkIII: 2,250 Knowledge, 30 cap, 500 energy.
(Remember the MkIII unlock allows the special MkIV if an advanced factory is captured.)
I think it's fair to say that the Knowledge cost of a stationary radar tech can't be higher than 2,250. I think 750 or 1,000 would be ok. We need to adjust the rest for something balanced with this target K-price.
Cap and energy are, IMO, aiming at the same goal: limit the use and force choice. We can debate that if it's limited by supply to the adjacent frontier, the limit is already put. However, if unlocking this tech would just grand free vision on the frontier (which is nice), with no galaxy cap and low energy cost, it would cost around 2,000 knowledge, which is near the price of the MkIII scout that may allow the same outcome.
My proposal is then to have a lower limit, a lower K-cost and an interesting alternative to the MkIII scout. Then we can give the radar a low galaxy cap and/or a high energy cost. We can do both but, while the K-cost may drop to 500 (maybe 250 or free), it wouldn't be a viable tool.
The first possibility is high energy cost, high cap (or no cap). I believe no galaxy cap (let say a cap of 1 per planet to avoid cheese) and 5,000 energy cost would be very interesting because it scales with the galaxy size: the larger empire a player has, the more (s)he's likely to have enough energy to spare.
The second possibility is low energy cost and a galaxy cap. 500 energy matches the MkIII scout and sounds low enough; a cap of 30 sounds reasonable for a whole frontier. However it won't scale with large galaxy or large empire (or thin empire with lot of adjacent enemy worlds). A higher cap would be silly because it would be like no cap at all, considering the supply limitation.
TL;DR:
My favorite solution is:
* Requires supply (survey only the frontier)
* No galaxy cap (but 1 per planet)
* high energy cost (5,000 is my proposal)
* low knowledge cost (between 500 and 1,000 according to what you think of the energy cost)