All convoys in pak128 are balanced using a formula written (probably as a spreadsheet or something) long ago. Zeno made a program from this so its program can be used to balance your own convoys. All French addons, except for archived addons and MLM dev. maglevs, are balanced using this. The calculations done here are complex so extracting a formula from them would be overcomplicated. This program has a user interface, you can load several .dat files in the same folder at once, although it can't load .dat files containing multiple vehicles (so you have to keep one .dat file for each vehicle). After having loaded your .dat files, find the tool "balance openpak128" (hidden in a menu), then save your .dat files, and it's done.
There are apparently formula used to derive costs for convoys. These are likely inaccurate and in the end you probably have to resort to manual balancing.
What you have to keep in mind considering pak128's economy: it was neglected for a long time, resulting in a bunch of inconsistencies and many vehicles being "cheated" compared to others. I think Zeno's work is not intended to be the perfect economy setting, but it still makes most vehicles balanced between each other, it is already a big step forward. In my opinion the current drawbacks of pak128's economy are that it works well between 1930 and 2020, but won't work well outside of this period; profit on passenger's transportation depends too much on speed so that in middle and late games buses can't be profitable (but still necessary for the rest of the network to get passengers) and commuters are too little profitable, so you don't have the choice, you have to make high speed lines (trains or planes).
Anyway, until now, all SNFOS addons (in the "right" era) balanced with Zeno's program are playable, we had not to rebalance them by hand. So it should be okay for your addons too.