I've used per tonne prices of $ 18
for ilmenite, $ 100 for rutile, $ 133 for zircon, a 50 % haircut for probable reserves, and 75 % for measured & indicated resources.
I must say however, your «medium term prices» are unrealistically low and I can not support your reluctance to use current spot prices
for ilmenite, rutile and zircon.
I'll be kind & use the same in - the - ground prices as last year (approx. 10 % of medium / long term average spot prices): A price of $ 18 per tonne
for ilmenite looks spot on, the $ 100 price for rutile's mostly irrelevant, and a $ 133 price for zircon's generous considering recent grading & pricing.