Times with highly increasing prices is no news. In my experience, what you have to do is several things at once. You will probably never hit near the correct answer by mathematics only.
A few suggestions:
Ask supplier for current prices.
Ask for validity time for the price.
Ask for prices if supplied next year, or the suppliers thought on price increase for the next year/periode.
Split your calculation in so many parts/details as practical possible.
Add an extra post for price increase for each sector, like current price for piping category Post A, and separately expected price increase for Post A, one year.
Always present your calculation with a probable accuracy (Less 5 plus 15 percent????
Add an extra post for unexpected expences (10%)
For each post rethink: what is in worst case the highest cost? What is the absolute lowest? (The truth is in avarage
somewhat in the middle)
In this way you will be more able both to argue for a realistic budget, and the single uncertain factors. If you are forced by internal or political reasons to reduce your calculation, you will at least not be blamed if your point later shows to be more correct.
An offer will always reflect not only present material prices but also present activity. High activety -> prices offered will be higher, and it will be harder to bargain reductions.
Also remember your plain statistic: some companies, organizations and single persons always land up with a 'systematic statistical fault',: always calculating too low.
A good cost engineer will on an average present correct calculations, and if a systematic fault is introduced, it is easier to dispose of extras in a budget a bit too high than find mony in a too low budget.
