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.
You are absolutely right that the floor will move independently of the walls. The movement when an adult steps in is not as much as a full bath of water, say, but it is still enough that you need a flexible seal of some kind to avoid water pooling and mould growing.
I asked a similar question which @ChrisF answered very well which suggested that caulk should be flexible enough to cope. That is what I did, and so far I have had no leaks (I have taken the panels off to check every now and then)
Best Answer
No slope is a bad idea - sure, you can have the joy of vacuuming up water on the platform after every shower or bath...no, really you want drainage.
If the tub (as usual) stays put, you can set the tub level, and slope the surface around it (or under it, other than where the feet sit, if it has feet) to drain.
Honestly, my first thought here is that the tub base/surround should be a non-raised extension of the shower base. A step like that invites falls in the bathroom. You step out of the tub, it's further down to the shower floor than your brain expects, or you slip off the narrow step, and hurt yourself.
It should still be sloped to drain, while the part exactly under the tub should be level (and full of plaster, since I'm old-school that way with making plastic tubs be solidly bedded. Tubs that flex, crack.) Either raise the side towards the drain to be level, so the tub sits on a very short plinth, or let the surround base & tile flow up the skirt of the tub to embed it the half-inch to an inch it might need on the high side.