I want to know how you feel about balancing units in maps with a separate technology tree. Is there any theory behind it or do you always follow your intuition?

Let's say every player starts with a technology that allows him to build level 1 units. He has an option to advance his technology which lets him train level 2 units. Now, how do you balance that? Should both types of units cost the same amount of money? Or, should the level 2 unit be just as strong and be just a container for more firepower. What I mean is:

2x Price = 2x Stronger? (of course, price including cost and time of the technology)

Slightly stronger, weaker? - how do deviations from that scale in any direction affect the game play? If they are weaker than 2x2, then there is no point to advance your technology. On the other hand, I made a map once that had units a bit stronger every time they advanced. Players didn't like that. It was fun for the first player to up his technology, but not for the rest. (Have you ever played the civilization map? Usually, with less experienced players, it's always the same - players race towards one given technology that they feel confident with and then make some military movements.)
Another solution that comes to my mind: keep that 2x2 proportion but make units more skillful - increase range, better attack type, etc. Then again, these are just less evident damage and hp modifications.

Obviously, there must be some kind of 'goodnes' ratio that takes cost, time and global importance of technology into account. Have you worked out any formulas for that? Any hints? What does the average player like the most? What makes the game enjoyable?

imo, this is y i avoid playin' any time of madness maps because of the tendency of units to be overwhelming. if you want them to tech up, lower tech units should still be able to kill higher tech units. the only example i can think of evolves panther where u can still kill units higher up but at a lower clip, not too low where it seems insurmountable. i hate balancing units, there should be a fixed formula.

ie. zergling has X health and does Y damage, marine should have X health and do Y damage

Balancing is at times tough to do, because you need to consider lots of things. Range, damage, attack rate, cost, health, and allowing some units to beat other units. X beats Y, Y beats Z, and Z beats X. A basic triangle, but micro can make it so X can beat Z. Adding spells makes this system a whole lot harder....

When ever I need to balance, first I think about how the game is going to be. (Let's say it's a defense map.) I first make a standard unit (The one all the other are based on.) Then from that, I make the others. (If Hydralisk got 20 HP and 5 attack, marine should have 25 HP and 5 Attack. [This is only the first draft.]) All I think first is how much hit should it take to kill a unit and how much time it takes. After I'm done, I test it. If it works well, then that's good, if it doesn't, then I can change it a little (change the armor for example). Also, it's a good thing to consider the amount of each unit that will be used. 10 marines versus 30 hydra can do a big difference. The more you do it, the easiest it becomes since you know more about the little details of each units.

Balance should not be a reason to stop working on a map.

Hope that helps in someway.

... loooots of play testing