The Survivor balancing issue

Hello everyone,
it turned out to be quite hard to find a working Survivor mode balancing so I want to try to describe the issues we have. Together we may be able to find a solution suitable for all players and map sizes.
About Survivor mode: Traditional TD games that are providing a Survivor-like mode are usually built on the concept of creep an round bounties. This means that a player receives credits for killing creeps and for finishing a round successfully. The amount of credits is fine tuned to the specific map and fixed waves in this cases. However, in CreepTD there is the "Income" that raises credits exponentially on the one and the different map depths on the other side. Both these features make it hard to find a send strategy that's suitable for every case.
The approaches we tried till now have been based on the AI's income and the credits the player receives. But it turned out, that all these approaches have been too easy or too hard because of the following fact: Every time the server sends creeps, its income is raised. Every time a player kills creeps, its credits and/or income are raised also. So: If the AI has more income, the player has too (when killing the creeps). So it doesn't make any sence to adjust the amount of credits or income the AI spents when the client receives this later, too. Even when aggressivly raising the Survivor's credits, the player usually will not be able to defend.
Because of this the only factors for the game's difficulty seems to be some sort of limiter for the client's credits and/or income. Maybe some kind of "tower upkeep" (e.g. 10% of its price/round) or similar. Maybe some sort of fixed income depending on the map length. Unsure.
Any ideas?
it turned out to be quite hard to find a working Survivor mode balancing so I want to try to describe the issues we have. Together we may be able to find a solution suitable for all players and map sizes.
About Survivor mode: Traditional TD games that are providing a Survivor-like mode are usually built on the concept of creep an round bounties. This means that a player receives credits for killing creeps and for finishing a round successfully. The amount of credits is fine tuned to the specific map and fixed waves in this cases. However, in CreepTD there is the "Income" that raises credits exponentially on the one and the different map depths on the other side. Both these features make it hard to find a send strategy that's suitable for every case.
The approaches we tried till now have been based on the AI's income and the credits the player receives. But it turned out, that all these approaches have been too easy or too hard because of the following fact: Every time the server sends creeps, its income is raised. Every time a player kills creeps, its credits and/or income are raised also. So: If the AI has more income, the player has too (when killing the creeps). So it doesn't make any sence to adjust the amount of credits or income the AI spents when the client receives this later, too. Even when aggressivly raising the Survivor's credits, the player usually will not be able to defend.
Because of this the only factors for the game's difficulty seems to be some sort of limiter for the client's credits and/or income. Maybe some kind of "tower upkeep" (e.g. 10% of its price/round) or similar. Maybe some sort of fixed income depending on the map length. Unsure.
Any ideas?
