I can speak about turbidimeters as this is instrument I've designed few of (as in 2 commercial products). Getting into 30 USD would be hard if you want any performance out of it.

Just source of light is going to be some kind of NIR laser, VCSELs are getting cheaper, but it is still nowhere cheap as LEDs. Then you are going to need some form of amplification for your light sensor and most probably two of those. Then some ADC chip and related circuitry, power section and so on.

But thats just a board, you will need enclosure and you want something better than just a generic box as you are going to deal with stray light. Also add some vials so users have something to put the liquid in.

We are talking about DIY builds now, but if you want to sell those: you need to budget for packaging and sending products. But there are also going to be problems: some devices are not going to work or die soon, you will get returns, some products will get lost and you will have to pay for it.

It is dollar here, few cents there but those will add up quickly.