Affordable as in how much overall?

The cheapest junk you're going to get is $30-40 headphones from GC or somewhere equivalent and a Behringer headphone amp.
But that will suck.
Minimally you want to fork out a bit more than that.

Just start throwing out options. I've hopefully got up to $500.

Mackie HMX-56

Sennheiser HD 280
or
Beyer Dynamic DT770

Since I'm still new in the recording world, what is the advantage of having the 4 TRS inputs for different instruments? Wouldn't you take care of the dynamic balance on the mixer already? Or is it to single out to each their own?

The point is that you can have 4 different inputs besides the master. This allows each person using the headphones to dial in their own mix. The drummer can turn up the bass and guitars and the guitarist can turn up the drums but down the vocals.