I think as long as your espionage level is high enough you can spy on them all you like without actually having to place any agents.

I have to say I like both systems from Dread Lords and Dark Avatar, The Dread Lords I could just pour money into the system and they are spied on without any spys needed.

That changed in Dark Avatar, You Have to now place spies to gather inteligence on your enemies. While placing spys is good and slows down an enemy you also have to keep producing spys to keep up the inteligence as well as kill enemy spys. The only unwanted side effect is that I like to spy on some friendlys but not reduce the tile production on an friendly planet and for that there is no option and I would prefer to save my spys to target those I want to target rather then target the freindlys just for inteligience and get them wiped out.

I have to admit I would prefer a cross between both where I can pay money into inteligence to just get some information only and spend money on a spy to reduce the tile bonus or even steal a technology.

TA will keep DA's system, but will give the option to use a spy on a civilization directly.

There's a wonderful building that will take care of that issue for you. A bit on the expensive side, but then so are spies, aren't they   CE buildings will keep spies off your important worlds, and the AI tends to go after stock exchanges anyway - you may as well leave those spies in place late-game, as removing them costs more than what you're losing to the disabled stock exchange.

As for the rising cost of spies, it's an absolute necessity. Otherwise, if spies are fixed in price, they will be unobtainably expensive early in the game, go through an reasonably affordable phase midgame, then become dirt cheap late game, to the point where you may get several per turn. You might make an argument for adjusting how fast the cost grows, but growth is necessary.

I like the idea of offensive vs defensive spies, but even better would be a passive counter-espionage tech line. Each tech may give a 5% chance of passively removing a spy each turn, up to a maximum of maybe 20%. Have the counter espionage building available after the third or fourth tech, and still allow spies to be used defensively when necessary. (Sure, the passive tech would remove the spy from your 700% bonus industrial sector eventually, but who would really want to wait?)

Great! This was exactly what I wanted. Now the espionage system will be perfect!

I think things would be better if the increasing cost was a function of time rather than a function of the number of spies previously produced. Have some sort of global spy cost that is the same for everyone at a given time and increases over the course of the game. You could have espionage techs that reduce cost based on the global baseline.

Heck if you wanted to get really fancy you could make it sort of a market-based system where the cost per spy is dependent on the number of spies currently in the game globally.

Espionage is a bit odd in DA - many instances I wanted to spy on a neutral/friendly without doing harm or provoking them. (When a spy is detected is it ever known who is doing the spying?)

While the rising cost is needed to prevent massive amounts of spies and micro-management late in the game while still allowing them early in the game, I think there's a better way to do it. Instead of spies costing more over time just give each spy a signifigant cost to keep up. Or a compound upkeep that increases with the number of active spies you have. To keep the spies from popping out automatically and creating an economic drain in the background, make it so the designated amount of money put into espionage via the espionage slider is the total funding - thus halting training of new spies when total espionage upkeep equals that amount.

I'd like to see a system where pro-active spies can still be placed actively to steal tech or sabotage, but have the pool of spies not placed in enemy territory work to nullify enemy spies and to gather general information on all races. The more passive spies you have available the more detail info you have on other and the better chance you have to find and nullify enemy spies with less chance of losing one of your own in the process.