What is the difference in cost of making a use-activated item cost aswift-action rather than a standard-action.
I.E. a potion/whatever cost a free/swift rather than a standard?
Generally the only way to make a spell effect swift would be tacking on quicken meta. However that doesn't work with potions as the rule is standard action to drink. I believe in 3.5 there was a masterwork potion belt which made up to 6(?) potions move equivalent to drink. In PFRPG there is that trait/feat that allows you to drink things faster as well (away from books).
RAW PFRPG, the best I think you'll get is a contingency to cast a spell with the spell effect of the potion you want. Casting of spells costs are covered in the trade goods are I believe. Anything beyond that as an item, you're getting into custom wonderous magic items, which is up to the DM so you need to ask them.
Due to action economy, making quickened items is up there with continuous truestrike effects. Basically much more than you want to spend being the answer.
The cost is infinite... ergo... it's not something priced out for a reason.
High yes, infinite it is not...
What is the difference in cost of making a use-activated item cost aswift-action rather than a standard-action.
I.E. a potion/whatever cost a free/swift rather than a standard?
Per rules-as-written, impossible.
There is a reason the rules-as-written don't allow you to do this. You should seriously consider the implications and possible abuses of allowing magic items to be activated as a swift action.
As a baseline, a meta-magicked quicken 1st level spell is the equivalent of a 5th level spell. So, even if you hand waved the third level restriction and the whole activating debate it would be 2250 gp (5 x 9 x 50) for a potion of quickened cure light wounds. How many do you want to buy?
MIC for 3.5 had these. Base your costs ofd those?