More expensive than the upgrade...

Just as a note for anyone considering buying these in the Bazaar, consider Forgotten Spidersilk Slippers -- the upgraded Shadowy+9 slippers, which are BIS (Best in Slot) -- instead. Those are available from the Rat Market for 3000 shillings (the equivalent of 300 Echoes) and, as always, you can sell various items at the Rat Market at a markup from Bazaar selling prices, which effectively discounts the 300 Echoes further.

For example, you need ~5.1 Parabola-Linen Scraps to buy the S+8 Hushed Slippers, but just ~3.5 Parabola-Linen Scraps to buy the S+9 Forgotten Slippers.

The down side is that what the Rat Market buys and sells is random, so the option may not always be available.

See the Rat Market Guide for further details.

If you already have Hushed Slippers, though, note that they can be upgraded to Forgotten at the Cave of the Nadir.