I never used to QT but after having an ich breakout which took mu multibar angel ( a delicate species that was healthy and eating), My flame wrasse harem( 1 supermale 1 sub male and 4 females), my copperband, my muelleri butterfly and regal angel, I vowed never to not QT a fish ever again. It was a $1000 learning lesson.
If you factor the price of a decent QT tank, it is not much compared to loosing the fish, whether the fish is cheap or not.
My QT set up is as follows:
30 Gallon tank -$30
150 watt heater - 25$
aquaclear 50 - $50
some live rock - 0
DIY stand from spare 2x4-0
clip on light - $13
bulb - $6
Thermometer - 5$
That's about $100 compared to loosing a $1000 worth of fih, factor in the headache and time spent getting all the fish and treating them and catching them etc, IT'S JUST NOT WORTH IT NOT TO QT.