If you look at p24 graph 13 of following link it shows the US$...

If you look at p24 graph 13 of following link it shows the US$ thermal coal cost curve - A$40-50 (US$30-US$38) cash cost would be one of the lowest cost coal mines in the world (on a quality adjusted basis)  - http://www.rba.gov.au/publications/bulletin/2015/jun/pdf/bu-0615-3.pdf

The key variables in the cash cost will be the strip ratio, yield and kcal (washing, rail, port, tax etc should be easy to estimate).  If someone knows broadly what these could be then I could come up with an estimate.

Look at p8 & 10 of the old Aston presentation - - http://www.asx.com.au/asxpdf/20111003/pdf/421hncxl4fkwtg.pdf .  

Yes costs were higher then and there is some old equipment at the mine from 2006, but they haven't come down so much that it now would only cost $15-25m.  Clearly the AZT mine is a bigger mine, but these costings give some context given it is nearby.  BTW - if you outsource the capex you then need to pay back the cost of the capex in the opex i.e. you can't have super low outsourced capex and super low cash costs by not taking into account the return the capex provider will require.

Also, a 5mt mine is quite a big one.  Plus you need to determine if this is 5mt ROM or product coal.  The yield will determine the difference and with thermal coal it can be large (ash is a key component, although I believe NSW coal is reasonably low in ash).

Again, I think it is exceptionally unlikely Anglo would be selling a super low cost mine or would have put it on C&M in 2006 if it only cost $15-25m (likely less in 2006) to convert to a super low cost open cut.

This is a punt on thermal coal prices stabilising and grinding up over the next 12-18 months with a proven money maker in Tinkler.  If thermal coal prices don't go up, it is hard to see a sustained rally.