I took this off the Illinois tax website:
Tax Rate
The rate is 6.25% of the purchase price or fair market value,
whichever is greater.
If you acquire the aircraft by purchase from a person or business
that is not in the business of selling aircraft at retail, you must pay
tax on the aircraft's purchase price. If the aircraft's purchase
price is less than the fair market value, you must pay tax on the
aircraft's fair market value on the date acquired or the date brought
into Illinois, whichever is later. If you acquire a share of an
aircraft, tax is based on the purchase price or fair market value of
the share acquired. There is no provision for a trade-in allowance
under Aircraft Use Tax. The tax due is based on the fair market value
of the aircraft or share of the aircraft.


I like the words "which ever is greater"