To get a low valuation why not use the valuation airlines themselves use on their financial statements, which is about 0.1c/mile (actually about $23 per 25,000 miles for Delta) ?

I would argue that since the IRS allows only $0.001/mile as a liability for the airline tax return, they should not tax you at a higher valuation.

Most valuations based on actual economy class tickets overvalue frequent flyer miles, since paid tickets are not subject to similarly strict capacity controls. Thus even if I value them at $0.01/mile when I have a choice between a FF ticket and an actual ticket, many times I need to be much more flexible with dates just to have a chance to get a FF ticket, and any fair market valuation must take into account a discount for this loss of flexibility.