PCB Tg 150 vs Tg 170
Tg, Glass transition temperature, is the temperature at which the laminate material losses its properties changing from a glassy state to a rubbery state. 

It is known as well as Heat distortion temperature.

As general rule the higher the Tg, the more stable the material is during the PCB manufacturing process and assembly. As well, the higher the Tg, the more expensive the material is.

For FR4 materials this parameter is between 130°C to 190°C. Other types of materials can go up to 260°.

The main aspect to consider here is the PCB complexity. High Tg materials are required for complex multilayer PCBs.

For a small 4 layers PCB, FR4 material with 150°C Tg should work.

If you are not sure about what material you should specify, ask you PCB manufacturer. They know their process and they know what they are capable of.

Anyway, it sounds like a big increase x10 more expensive 170Tg than 150Tg. I suppose that's their preference and they are not really keen on working with 170Tg material.