There are a few techs that have slightly varied costs depending on the civilization, but I'm not aware of any differences of the magnitude that your experience suggests (aside from NLCs for Torians).  It may have more to do with TA's tech-count inflation...at certain points, inflation rises from 1x/2x for non-weapon/weapon to 1.4x/2x, then to 2.1x/4.5x, and finally to 2.8x/6.1x.  So if a civ goes for weapons immediately (or planetary invasion), it'll cost them much less than it would if they wait until the 30th or even 50th tech.

Warp III isn't that hard to get if you make it a priority.  If I didn't have more important things to get, I'm fairly positive I could get it by turn 10 in almost any tree.

The AI bases the value of the extreme techs on how many (uncolonized? can't remember) of those type still exist-but I never bothered to see if there was a modifier for Barren for Yor (Isolationist) or whichever two Adapter has or Toxic for Korath (Spore).  Note that this applies both to selling it to them and buying it from them.

Sidenote: PI and successive techs count as weapons tech, but defense techs do not, for this purpose.

I don't know what motivation the Altarians might have had-perhaps the Drengin were raining troops down on them?

-

Tech trading is the ability to trade techs that you own to another civ.

Tech brokering is the ability to trade all techs that you own; turning it off means you can only trade techs that you have researched.  So if I have tech brokering off, and I trade for PI from the Korx, I can't go trade PI to anyone else-but the Korx can.  It works kind of like trade goods, except it doesn't prevent other civs from researching the tech (and thereby being able to go trade it).

2.0's spy system changed how you can raise your espionage level, but you still need agents on a planet to siphon technology (which, by the way, agents on a planet no longer increase your intel level, either).

-

What difficulty are you guys playing?

Players used to complain about the AI group-researching, but disabling tech brokering mitigates this issue to a large degree; not to mention that the AI shouldn't be that intelligent below at least Tough.

Which techs might these be?

I've always found that turning tech trading/brokering off will more stiffle the player than the AI. Esp. in TA where you can have all AI techs for an apple and an egg....

Not silly at all. You will find that the stupid AI leaves planets undefended from time to time and if you have allies they will take care of defenders leaving planets open for your transports. The invasion technolgies also boost your soldiering bonuses i.e. your planetary defense and IMO one should build up a good defense first before worrying about offense (ships and weapons). Having large populations of well trained (and tax paying) soldiers on your planets is also a far more economic defense than having a ton of expensive defenders that take time to build, cost maintainance and constantly get shot down and have to be replaced. If the enemy transports become overwhelming or you just want to minimize your losses you can use fast small ships with a single weapon to take out any undefended transports approaching. You'll also find that the AI players are keen on researching and trading the different weapon and hull techs so with a bit of diplomacy and perhaps some espionage it should be easy to obtain those techs from them, enough for you to focus on building up a strong infrastructure and outclassing everyone else in terms of research, economy and production.

Things that they shouldn't have in the very early game, like advanced weapons techs, or planetary invasion techs...in the same time that it takes me to research four or five very basic techs, the AI in the diplomacy screen will sometimes have a list of techs a mile long.  there's just no way they could research them all the same way that the human players have to.