I'm sure if devs were working for you they'd be very happy to be on an average salary of £125k. From the 2018 yearly
report:
So the entire spend was circa £16m for 400 staff (as per
official Wikipedia). Likely to be somewhat less when you strip out research costs.
That's roughly an average of £40k per staff member. 100 staff* x 40 = £4m a year, so circa £8m for two years. That's about $10.5m.
If each expansion is £40 ($52) they'd have to sell roughly 200k to break even.
* of course many don't believe 100 staff are working on the game, so ironically the costs are going to be less.