Presuming you are referring to US domestic airports, these are almost uniformly employees of contractors to the airport authority and are rarely paid much above minimum wage. This is a simple way of saying that they depend on tips to subsist. Depending on the airport, getting to/from may be quite expensive (plenty of recent newspaper stories about how little these folks make).

How much depends on the service and how much time is involved. One wheelchair pusher who spends 30 minutes with you ought to get $10+. A cart-driver who spends 5 minutes with you and 4 others ought to get $2+. Maybe a bit more if they are handling luggage as well.

There may well be 200 frothing-at-the-mouth posts about why this is wrong, but that is the way it is.

That said, if you require assistance and really cannot afford it, you should tip what you can afford, bearing in mind the above. Also important to remember that disability assistance is a right. Skycap substitute is not.

Bottom line is that you pay what you can and "expensive" is relative.