An external source fixes the price of advancements of things not yet added
 

In the Dark Heresy game, the cost for an advancement is set by the career your character has chosen or by the ranks that career allows you to choose. For example, a Guardsman's first advance in strength costs 100XP, the 2nd 250..., but a Scum pays 500XP for the first and 750 for the second.

The attributes I have working - since the attributes are always bootstrapped to a character, I can have the career selection assign Advance1.100, Advance2.250, etc. tags to the strength attribute, and then have the advance find those tags and calculate its cost.

It's the talents I can't figure out - Talents aren't automatically bootstrapped to the character, so how can I assign a different price to each talent? If I use childfound to assign the tag, will it be added to an advance's child in time to calculate the price?

Okay, I guess I found another case where the process of writing out the question led me to the answer to the question. Using childfound to attach the tag worked.

You can assign tags at any time to any pick or container. You can also set field values. Using transitions, you can access virtually anything within the character from anywhere else. So you can readily orchestrate the proper assignment of prices to the various talents.

You (as author) are completely in control of the timing of all Eval scripts. All pricing is calculated via Eval scripts. Consequently, through the appropriate selection of phases and priorities for your Eval script, you can ensure that tags are assigned before any prices are calculated. If you want to be extra safe, you can also define timing dependencies to your scripts, which will ensure that HL traps any failed dependencies if you modify the timing of those scripts in the future.