The *base* cost of retro seems to be capped after your 13th visit (incidentally the visit which gives you the "Stop! Hammertime" achievement).
i.e. base cost is $100 per visit, up to $1300 on your 13th, 14th etc... visits.
The same multiplier still applies for retroing before 20 weeks has passed.
Putting this in research before I add it just to get someone else's experience.
I don't think I'm wrong though, I'm now on roughly my 20th retro and cost is still 1300.