There is no way of knowing exactly what you should use without looking at the specific unit's characteristics and running them through the algorithm to build a specific unit AI type, and then doing the same for some other units and comparing the values. Different unit AI types produce values in different ranges and consider different factors.
(To just see a general recommendation for a starting point, the next to last paragraph gives one.)
Example: for the basic Attack unit AI type...
let CV be the land unit's "combat value" - this is based on the unit's strength and the number of first strikes and first strike chances it gets all ultimately divided by the
strength of the strongest land unit that any player has ever built in this game. It is, for a land unit,
CV = 100 * S * (((F * 2) + C) * 5 + 100) / 100 / H
where S is this unit type's combat strength, F is number of first strikes inherent to the unit type (not from promotions), C is number of inherent first strike chances, and H is strength of highest strength land unit ever built by anybody so far in the game. (So it increases the value by 10% per first strike and 5% per first strike chance.) Therefore if this is a unit that has a strength equal to the highest strength unit anyone has ever built so far and no built-in first strikes or chances, then it will get a value of 100. Weaker units will be proportional to that (50 for one half that strong, excluding possible first strike related increases). If the unit being evaluated is stronger than any land unit anyone has ever built so far then it will have a CV over 100. A unit with built-in first strikes and/or chances can also end up over 100 (especially if it has a strength that is equal to, or greater than, any that has been built so far).
the basic UNITAI_ATTACK evaluation gives a value via:
let F be 1 if the AI is not using the "fast movers" strategy type, 2 if it is
let M be number of moves the unit gets
let W be the unit's withdrawal chance
let L be the unit's combat limit
start with Value = 1 + iAIWeight
then via integer math (so all fractions are dropped from each step before applying them), to Value:
add CV
add CV * M * F / 2
add CV * W / 100
then if the unit has a combat limit under 100 (so it can't kill its target, like a catapult),
subtract CV * (125 - L) / 100
For the attack and city attack unit AI types, if the AI is evaluating the unit for an overseas attack and the unit starts with a free promotion that has the amphibious property then its Value is increased by 33% (dropping fractions).
Now you finally have the Value assigned to the unit for the attack type unit AI.
Note that this is the basic preliminary value. The evaluation actually runs through everything it can build twice. The first time to get the highest value, then it reevaluates every unit that is at least 67% of this value and considers additional factors that are the same regardless of the unit AI type being evaluated. All of these additional adjustments but one are multipliers to the base value. These factors include how much XP the unit starts with (+10% per free XP, rounded down), a bonus if the unit type gets one or more free promotions from buildings, another if it gets one or more from its inherent properties, and likewise from civics (working out to +15% per category in which it gets one or more free promotions). Each also gets multiplied by a random factor from 1 to 1.49 (in steps of .01), and also gets a bonus if it can currently be rushed without happy cost or if it can't be rushed with the bonus equal to the production cost already invested in that unit type, a penalty related to how many of that unit type it is already building in its cities, and a penalty if it dies when it attacks (suicide flag set), and finally another factor which depends on whether or not rushing causes unhappiness (or if it can't rush) to determine which of two adjustments to make. As you can see, it is somewhat complex and includes a significant random factor...
When being evaluated for a different unit AI type, the same unit will end up with a different value assigned to it except via coincidence.
So...
If you want a general place to start when picking a value for an iAIWeight, I'd suggest that values under 20 will probably have only a very small effect, if any. Values of iAIWeight around 50 would should have a noticeable effect but not hugely shift the preference, and values up around 100 should have a stronger effect but still allow other units to be built for that unit AI type, at least sometimes, unless perhaps all the unit types in question (those that can be built for the specific unit AI type being evaluated) are a lot weaker than the strongest unit type built so far. Values up over 200 should have a large effect and possibly (for some unit AI types) overwhelm the chances of building anything else for that unit AI type unless some other unit gets more moves or some other bonus (like built-in first strikes) or the unit with iAIWeight bonus gets some penalty the others don't (like not being able to kill its target in the case of the attack unit AI type). But, again, each unit AI type uses a different calculation, some with several more factors than the attack AI uses, and therefore the same iAIWeight may give a large shift when the unit is being considered for one unit AI type and a not so large shift when being considered for some other unit AI type. If you assign a unit an iAIWeight that makes it picked a more often for the city attack unit AI, it might also have the side effect of making it always be picked for the attack unit AI type too.
Perhaps giving an iAIWeight of 50 or so and seeing what effect it has would be a good place to start.
If you want to examine the source code where all this is done, the picking of a unit type for a specific unit AI type is done in CvCityAI::AI_bestUnitAI. The basic value generation of a unit type for a unit AI is done in CvPlayerAI::AI_unitValue.
