It will depend on how you are stepping into.

First way, and the cheapest one: you already have the same army for 40k and you are recycling a KT squad. Blood pact, Aeldari aspects, CSM legionnaries...

Second way: coinvesting in a starter and sharing with a friend, leading to some savings

Third way: no saving at all, grab a box and with a single one you get you squad a more or less 50 bucks. Dark ELdar court will eventually be cheapest, but do you really want to play these?

To be kept in mind though: KT may be low in investment unit wise but may require some / more budget for scenery. Starter and Kill zone from the early editions were far better than what is now proposed budget wise (compare for example Chalnath KT with current Bheta-Decima...)

On average it is usually one box of base infantry so they will be roughly the same price. Custodes as an example would be 3 models (box is 5), Grey Knights Power armor is 6 models (box is 10), While the Sister novitiates box is uses all 10 models but can use all the upgrades in 40k. It is when you build your Roster that it gest massive.  

thanks a lot @war! I have a little senary, but I'm planning to play at clubs, and I already have a sizable collection of models from many different armies, but unfortunately, I never picked up any rules or tokens. if I'm planning to play tau or tryanids, will this box be enough to get started?

https://www.amazon.com/Games-Workshop-Warhammer-Kill-Starter/dp/B09QXPRDVL/ref=asc_df_B09QXPRDVL?tag=bingshoppinga-20&linkCode=df0&hvadid=80333194313168&hvnetw=o&hvqmt=e&hvbmt=be&hvdev=c&hvlocint=&hvlocphy=&hvtargid=pla-4583932718215289&psc=1

That would be a good start as it gives you the rulebook, tokens, and measuring tools. You may need to get the 2023 Annual as well. 

That was the box I was going to recommend

You can find all the rules online for free (**tried providing a link, but the forum auto-corrected twice, so it must be against the rules to share) . Some of them (the compendium teams) you might be able make with models you already have. They're less competitive than the "bespoke" teams (teams specifically made for Kill team, having more special rules and generally more powerful) but they are perfect to learn to play with. 
The box you linked above is a super solid jumping in point, kommando's are a one-box team (vet-guard needs two boxes to get all the options) and both are still among the strongest teams in the game. 
If you wanted JUST the tools needed to play the game, the "Kill Team Essentials" box would have tokens/barricades/measuring tools that would let you play with your existing models.