Just like any other tool, how much you depend on it, and the results you are looking for will for sure play into what you can get away with and thus price.
I've owned Red and Blue Tig machines, and the Blue is significantly better. These days I sport a Miller Dynasty DX 210. It's very expensive, but also works really really well.
Before I bought it I tried the ESAB, but the machine just wasn't that great. The menu's had software glitches and the fan ran at full speed regardless if you were welding at 8 amps or 80 amps.
Anyway, here is what you care about
Inverter or Transformer. The Transformer tig machines are huge and heavy, the inverter machines are light and portable. Inverter machines usually have dual voltage input while you will need 220 service for a transformer based machine.
AC/DC: The AC models can weld aluminum as well as steel. The DC only models can only weld steel.
AC wave control: Some AC models can do a sine wave, or square wave, or change the frequency of the wave.
Duty Cycle. The higher the duty cycle the longer you can use it before it overheats. This isn't an issue with 4130 aircraft tubing, but when working with thicker aluminum, it matters.
Service/support. I wouldn't touch a harbor freight tig, but I've heard people getting decent service out of Everlast, and of course Miller and Lincoln are the gold standard.
So, a base model with 60% duty cycle at 200 amps that can weld aluminum but doesn't have any ability to control the wave can be something like this:
Which is pretty similar to the Lincoln Square wave 200.
A step up would be this guy that has 60% duty cycle at 250amps that has all of the nice aluminum features:
The Miller which is a much nicer unit, but not technically any more featureful is this one:
Personally I'd get a Powertig 255ext or the miller depending on budget and how much you depend on it.