The 75-300 III is an older lens with a design that goes back to the '80s. It typically comes in a variety of flavors (with and without USM, with and without IS), but the cheapie one you can find new these days typically sports neither USM nor IS. And the optics are relatively old. That's not to say it's a bad lens, but it's more limited than nearly any other offering out there, and you have to pay special attention to using it in the f/8 or smaller range, and that your shutter speed is faster than 1/focal_length to get the best out of it. And if you're limited to f/8-f/16 and you're at 1/500s or faster, then you either jack up the iso, or you live somewhere very sunny.

Today, the EF-S 55-250 IS is probably a nicer budget alternative to a 75-300 III. It's a digital-era lens (introduced 2003), with an APS-C crop design which means it can be smaller and sharper than a 75-300 III and stabilized, for not much more.

The EF 70-300 IS USM is from 2005, and was basically the design that succeeded the 75-300 III IS USM. Its optical quality is actually on a par with the 55-250, but it has more reach, it's a full frame lens (i.e., can be used with a 5D/6D), and has USM. The expense leap is probably due to the additional reach and having to build a lens with an image circle large enough to cover a full-frame sensor. Crop lenses can always be a little less expensive because they only have to cover an APS-C sensor, and require fewer and smaller elements.

Also, be aware there are two other 70-300 Canon lenses, with pricetags that will probably look insane to you: the 70-300 L, and the 70-300 DO. The L was created to combat all the complaints about image quality on the 70-300 IS USM, which is still a middle-grade lens, not a pro-grade one. The 70-300L is more of a little brother to the 100-400L, it's white and sports a four-figure price tag. The 70-300 DO was an attempt by Canon to create a smaller telephoto lens with "Diffractive Optics", and only two lenses were ever designed this way: the 70-300 DO (which most folks will say isn't worth the money), and the EF 400 f/4 DO IS USM, which wildlife photographers prefer when they have to haul their gear across the frozen tundra for miles.