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$. A portable hot tub around 2,000$ and an inflatable hot tub for around 400$.
Take those values with a grain of salt, as with everything, the price depends on a lot of factors.
If you are on a budget, then consider buying an inflatable spa. For the price (usually around 400$) 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.