Quick summary: A built-in concrete hot tub usually costs more than 10,000$, an acrylic one between 5,000$ and 10,000$. A portable hot tub around 2,000$ and an inflatable hot tub for around 400$.
The reality of hot tub prices is much more tricky. Whatever the hot tub type, the price mostly depends on these factors:
The size of the hot tub
No wonder bigger hot tubs cost more. More material used for manufacturing, more piping to do, stronger heater needed, stronger water pump needed, bigger manufacturing area needed, more expensive molding equipment, and the list could go on and on about why bigger hot tubs are more expensive.
This rule applies across the whole range of hot tub types. It's less pronounced with inflatable hot tubs, but that's also because the size differences between inflatable hot tubs are quite small.
The quality, design and engineering
With more expensive hot tubs, manufacturers don't have to save money by sacrificing quality (but some still do!). Better quality of molds with acrylic hot tubs, better plumbing, or higher quality (or more layers) of PVC material in inflatable hot tubs.
Don't forget about smart engineering decisions. There are hot tubs which pumps have impeller problems due to bad design decisions, which would likely be solved by hiring better engineers (yeah, that costs money) - or using higher quality materials (costs money too).
The design of the more expensive hot tubs is usually top-notch. By using more expensive materials, manufacturers are able to give them a more luxurious feel. We all know the difference between cheap toy-like plastic and a quality acrylic mold. The more you spend on your materials, designers, and engineers, the better will the resulting product be. With hot tubs, this is 100% the case.
One example is a wall of an inflatable hot tub. The high-quality Intex PureSpa has a super sturdy wall (great engineering and materials) and the finish has a premium feel (lovely design). On the contrary, a no-name inflatable hot tub we reviewed had walls that acted just like the inflatable toys you know from childhood (it didn't hold shape at all, you couldn't pressurize it properly as it would bulk out), and the material felt and looked very cheap.
Hot tub features
Another parameter that impacts hot tubs of all types. How many jets does the hot tub have? Are those airjets or water jets as well? (more jets = more plumbing, water jets = more expensive water pumps instead of air blowers).
More expensive hot tubs often include built-in underwater lighting, more effective (and more expensive) water treatment systems.
Do you want to have saltwater in your hot tub? Guess what, saltwater is super corrosive - manufacturers will have to use more expensive materials again.
Are the water jets in the hot tub adjustable? Does the hot tub have a digital control panel, remote control, or even a wifi control?
All these (and many more) features greatly affect the price.
This is an important thing to keep in mind when comparing different hot tubs and their prices.
More expensive hot tubs often come with beverage trays, seat cushions or stairs included. What about a cover, is it included? Expect having to purchase a lot of stuff separately with the less expensive models.
Usually not a problem with portable or inflatable hot tubs, but definitely a thing to consider when purchasing bigger hot tubs.
Acrylic hot tubs can easily weigh 600 pounds - and shipping that across the whole country is no cheap feat. Look for a local supplier when on a hunt for a bigger and heavier hot tub.
If you are on a budget, then consider buying an inflatable spa. For the price (usually around 400$) you actually get quite a lot.
Most of them are made of triple-layer PVC and are really sturdy. Not like the inflatable toys you might be used to. The inflated wall is so strong, you can actually sit on it.
Most of the inflatable hot tubs have the downside of not having real hydro jets that built-in hot tubs always have. Instead, most of them have bubble jets that shoot air. (But there are exceptions: this one has real hydro jets).
Another kind is a portable but non-inflatable hot tub. They range from smaller and lighter ones similar to blow-up hot tubs, but instead of inflatable walls, they have a fixed construction. The cheaper ones also have just bubble jets, but there are more expensive with hydro jets available.
In the range of few thousands of dollars and higher, there are still portable (or rather, not built-in in a permanent manner) hot tubs available. They are made of acrylic, and most have real hydro jets that shoot out water.
And the last and most expensive category are concrete hot tubs, usually tailored exactly to your needs and often built together with an in-ground pool. They can cost way above 10,000$ and are permanent structures.