MicroCenter has their Inland temperature color changing PLA 1.75mm 0.5kg on sale for $4.99 with free shipping. Also available for in-store pickup.
https://www.microcenter
Not the lowest price ever for filament, but good for color changing.
Edit: color changing sold out online, still available in-stores.
Also 2.85mm 1kg PETG on sale for $7.99: https://www.microcenter