The most common discrepancies in price between tubs, not counting accessories like heaters and jets, are due to four factors: style, depth, build quality and brand. A garden tub will generally cost you more than a simple rectangular built in, and a clawfoot is more expensive still, followed by "custom" installations like non-overflow tubs.
In the same style, the depth of the tub determines the volume and weight of water the tub must hold and therefore the dimensions and thickness of the material. More material == higher cost.
Even in the same style and depth, one maker may build their tub out of cast iron while another uses plastic, or even in the same material, one maker may double the thickness of the tub walls to increase durability and thus longevity (removing a damaged built-in bathtub is no small job; it can even require a full remodel because it requires re-tiling the backsplash).
Lastly, major name brands will command a higher price even if their product is inferior. Jacuzzi, for one, is a trademarked brand name that has apparently rested on its laurels; I was warned away from even their thousand-dollar tubs for quality reasons, but the tubs still cost a grand, cause they're branded with a universally-known name (so much so that people use the brand without knowing it's a brand, similar to "Band-Aid")
As far as actually what to look for, it depends on what you want. For an upstairs bathroom normally used by kids and guests, a standard Kohler, Moen or American Standard acrylic or cast built-in is fine. Cast iron is more durable, and holds the heat of the water well, but it'll be a beast and a half to put in place. For your own master bathroom, you can be pickier. The three brands I mentioned are names I trust, and they make tubs in all shapes, sizes, styles and pricepoints.
Cast iron tubs are very heavy and expensive. Rust is not a problem with cast iron tubs as long as the enamel top layer is intact (if you get a chip in it, you can get enamel repair kits). My wife did not like the old cast iron tub in our bathroom because it was cold when standing in it during a shower and made bath water get cold fast. Evidently she was not taking baths in the “cast iron method” which is to put super-hot water into the bathtub and allow the heat to transfer into the tub, this will cool the water down but keep it at a reasonable temperature longer (in theory at least, YMMV).
Enameled steel it basically a cheaper version of the cast iron, it is a lighter because the metal is much thinner. I doubt the “hot bath trick” described above for cast iron tubs will work in a steel tub. Like cast iron tubs, you should not have to worry about a steel tub rusting or cracking.
Acrylic (plastic) tubs come in a different quality levels. Some are really cheap, thin plastic that have the potential to crack over time (particularly if they are poorly supported/improperly installed). However some acrylic tubs use thicker stronger plastic (often with fiber glass embedded for tensile strength) that are very sturdy.
I recently had to make this decision; we replaced our cast iron tub with plastic Kohler model. I knew I did not want to lug a cast iron tub up to the 2nd story. I thought about a steel tub but they are still 120-150 pounds. I was worried about getting a cheap plastic one that would break so I settled on a $700 plastic Kohler tub that is pretty thick.
Best Answer
Best thing you can do is ask for references. Call some people who had their tub refinished several years ago and see how it's holding up.
I've had this done a few times and it's a bit of a black art. They have to be really meticulous with their prep work or the new finish won't last.
Also, you need to be careful what you clean it with after they refinish it! Abrasive cleaners will destroy the finish.