Coin-to-skill-EXP conversion rate using the 300 coin gairo draw

Duo

I am using the unlimited 300-coin summoning to convert coin to skill exp. I converted 100 times (2850 coins for 10 times) so the total cost is 28500 coins.
It gives me 1-4 star gairos. Usually I am only getting 1-3 star gairos, which are mostly used to convert to skill exp. The chance to get 4 star gairos is not zero but really close (on the order of 1/1000 I think), so it is ignored.

I summoned 100 times, getting 100 1-3 star gairos. This costs 28500 coins. I got 561 skill exp. Dividing 28500 coin by 561 skill exp, the conversion rate is about 50 coins per skill exp.