I would be much more likely to spend platinum at the foundry, if the cost linearly scaled with the time remaining.
I'd start building something, go to bed, boost it the next day and start using it.
I'd start building something, go to work, log in and see it has only a few hours left, boost it for the last few hours and get using the new frame/weapon right away.
But if it's a flat 50 platinum to boost the time for 1 minute, or 1 day, unchanging - there becomes very little reason to spend platinum that way.