Learn Farsi: اشتغال مهاجرها در مقایسه با سرانه کم است. - Migrants' employment is low compared with the per-capita figure.

Yes, they are highly interchangeable in formal or analytical contexts. You could easily say nesbat be sarane. While nesbat be translates closer to 'relative to' and dar moghayese ba literally means 'in comparison with,' both are perfect for replacing the simpler az (than) when dealing with abstract statistics or formal comparisons.

How does the word sarane translate to 'per capita'?

It comes from combining sar (head) with the suffix -ane, which forms adjectives and adverbs of manner or relation. So it literally translates to 'head-wise' or 'per head,' which is exactly what the Latin phrase 'per capita' means in English.

The English translation says employment is 'low,' so why does the Farsi use kam (little) instead of pa'in (low)?

In Farsi, when talking about amounts, rates, or volumes like employment (eshteghal), it is very common to use kam (little/low) and ziyad (much/high). You will also see pa'in (low) and bala (high) used specifically with the word rate (nerkh), but kam sounds perfectly natural here to describe the overall volume of employment.

Since this is an analytical sentence, why use mohajer-ha instead of the more formal human plural mohajeran?