To sum it up: A built in concrete hot tub usually costs more than 10,000$, an acrylic one between 5,000$ and 10,000$. Then portable hot tub around 2,000$ and last but not least, inflatable hot tub for around 400$.
If you are on a budget, then consider buying an inflatable spa. For the price (usually around 400$) you actually get quite a lot.
Most of them are made of triple layer PVC and are really sturdy. Not like your ordinary inflatable pool toys. Most of the inflatable hot tubs have the downside of not having real hydro jets and instead have bubble jets which 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 mostly have real hydro jets which 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. The can cost way above 10,000$ and are permanent structures.