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I know that, when a number declines, all of its components decline.
As such, the number 250.000 in instrumental would be двумястами пятьюдесятью тысячами.
However, I was wondering, would I ever need, in practice, to tell the number 250.000 in the instrumental case?
For example... "The price of our apartment was 250.000 euros". Does it make any sense to say "Цена нашей квартиры была двумястами пятьюдесятью тысячами евро"`?
In your particular example, 250,000 would be nominative to begin with.
To take an example where you'd actually need instrumental, though — say, "a suitcase with 250,000 euros in it":
чемодан с двумястами пятьюдесятью тысячами евро
— if you're a radio/TV presenter, maybe.
чемодан с двухсот пятидесятью тысячами евро, or something equally haphazard along these lines
— what you're probably going to get even from native Russian speakers in everyday life. We all struggle with numerals; it's an area in which Russian is currently shedding old complexity.
It's hard to imagine having to spell out such numerals in writing when you can just write the numbers, but theoretically, the same general rule applies: in a book or "serious" publication, you'll be expected to use the correct forms; informally, pretty much anything goes as long as it's some oblique form. If you look at the declension of сорок, it's more or less how all numerals will probably end up eventually.
