For me it depends on the brand. I know Opinel and Mora can make a good knife (not just decent) in the $10-$15 range; not every brand can pull that off.
I've only owned one Tangram knife, but despite being the budget arm of a brand I like and trust, Kizer, I wasn't impressed with that knife for $25. Kizer's Vanguard line, in the ~$50 range, is therefore where my floor starts with their knives (and those are excellent).
I'm automatically suspicious of any modern folder from a brand I've never heard of that's priced under $50.