x is a threat, will you declare war for 150bc?

A little exagerated most people and species would try to get a deal for the most they can get while trying to as much as they can for what they own.

not by far though. The ai is awful at asking for help in war. They almost always offer something absurdly low & want truckloads to do it themselves. The fact that I can't negotiate for techs and things is made all the worse. 

as to the cost, I didn't check what they actually want exactly & it's exaggerate yes, but pretty well reflects how they value military ships

This is one of those "the ai has different rules than the human" moments that plague the game.  Like asking for starbase removal or peace terms.

Generally, when asking an Ai to go to war, it will cost the player at least 3000cr in "stuff", whatever that may be.  Modified by relations, power, etc etc, mostly upwards. 

When an Ai asks a human, they offer 1 tech (usually the lowest end one you don't have) or 10% of their treasury (max) or 150 cr whichever is higher.  And there IS no option to negotiate.

So, if the game is going to assign value to requests like that, which it should, those values and rules should apply to everyone equally.  It's a simple matter to code and fix by just increasing the value of stuff offered and making it like any other trade deal.  No new mechanics involved.

Don't hold out hope this will get fixed soon though.  Diplomacy is getting reworked in the expandalone, so odds are it will get fixed there for $ rather than for free now.

I had wondered if it would be at all possible to be an arms dealer. It isn't. The only time I did much ship trading was when I was ahead in tech but behind in ships so I traded with minor races for their ships.

I doubt they'll put much into this feature, but the AI should value ships more if they couldn't build it themselves. Thus if I had bonuses like miniaturization beyond them I could make some money. I know I push my Hyperion shrinker pretty far.

One option would be to have levels of diplomacy attached to such trades.

For example just like when an event happens you have 3 choices, B/P/M, trade offers could do similar.

AI offers trade:

1. I will give you "X" for ... (results in high diplomacy trade value and value X is much lower than value being traded)
2. I will give you "Y" for ... (results in no diplomacy trade value - and value Y is close/near but still above the value being traded)
3. I will give you "Z" for ... (results in negative diplomacy trade value - and value is equal to (or just above) the value being traded) 
4. No I will not trade with you for any amount... go f.... (results in even more negative diplomacy - no trade performed).

The AI will then reject the trade based on:

1. Current diplomacy level.
2. how much it would cost for AI to get it by production/research
3. how bad it needed (hard to determine) but if at war, a warship is more valuable than scout, example.

Then as you build up trade diplomacy with faction you will get better value (for example friendly will more often go for #3(Z) trade while an unfriendly CIV will rarely offer/accept #2(Y).

150 bc is obviously a stupid waste of time.... under what circumstance could that be useful?

If your going to go to the trouble of developing such an AI diplomacy request in the game, then make it something that will test the decision making of the player, not waste the players time.

I though that was all AI have until i checking his treasury. Tosser was having like thousand in his pocket and trying to scam me into war wtih 150 credits. The deal that AI offer for most of it is fine except this one asking you to join war while paying you super cheap.