I think costs of 90 per ton is a bit ambitious, my estimate...

I think costs of 90 per ton is a bit ambitious, my estimate would be about 150 per ton + another 25 for shipping (in AUD) during the first few shipments.
Which would bring total costs to at least 175 AUD/t. however, once steady state production is reached you could expect costs to come down to around 120 per ton. My source for these values is from GWR announcements who is leasing JWD deposit to FEL. Their initial costs were 170 a ton and have brought it to 120 now.
Ramp up stage costs are often higher and will take a few hundred thousand ton to ramp up. Costs are higher than normal due to a tight labour market with many junior miners fighting it out at the moment.

I have seen many people on other IO juniors underestimate their costs and overestimate the profitability

Cheers