...The present value of any financial thing is what a typical market participant would pay for it now. In this case, the QTIP asset, I assume, is scheduled to go to a beneficiary or beneficiaries upon the death of your aunt. Upon that event, the deferred taxes would need to be paid, leaving a net residuum.
It sounds as if you're seeking the present value of $X to be received in Y years, based on a discount rate of Z%. You can estimate X using current tax rates and you can reasonably posit Y using actuarial data. Z is a market-driven figure. Is there a market for QTIP assets? If so, that market would reveal a range of discount rates. Higher rates usually reflect higher perceived risk in realizing the benefit; lower rates, lower risk. (NB: for IRS purposes, the federal government publishes tables of rates to use in valuing things like this, such as annuities, life estates, remainder interests. These rates are established for short-term, mid-term, and long-term situations and are pretty low. See link.)
I deal in real estate, so am not familiar with such a market; but I have valued mortgage notes as dealt in the secondary market, and also know that just about anything that has income or financial assets tied to it can be sold. Car loans, Luna carpet installment agreements, accounts receivable--you name it, someone out there will buy it, at a discount to face value. A CFA or CPA could probably help you.
Another factor to consider is that it's one thing to ask "what would the market pay for this benefit?" and another to ask, "what would the existing beneficiaries accept to sell this benefit?" If everyone's on the same page and would accept a market value, then market data can guide you. If this is a case where the beneficiaries weren't looking to sell their asset, then you might have to pay a premium over market.
The following is not intended as advice for you to follow--it's just my thinking out loud. If the assets are parked in a safe place, and the mechanism that relays the assets to the beneficiaries is clearly stated, and if there is no risk of the beneficiaries fighting among themselves in Y years to avoid honoring a sale to someone else--i.e., if all the buyer of the assets would need to do is wait for the Y years to pass, then the discount rate might be close to other types of safe investments apart from government issues.
You can find them on Google.
This should be a simple calculation given her age (then look it up in a life expectancy table) and an appropriate discount rate.
However I don't think you'd ever need the valuation, as (if I recall correctly) the estate tax pieces wouldn't trigger until her death, and the trust would be at the then asset values.
Just don't ask me what the calculation is.