A good starting point is to search the component on Digikey or Mouser to check the price and then when you have an idea of the price of the various components ask for a quotation directly to PCBWAY.
Like Dario said, LCSC has low costs. LCSC has great prices for passives(since they're produced in china) and has decent prices on some ICs, but many others(especially stuff like STM32) are very high in price compared to the normal prices in the US or other countries. You should pick one(or more) distributors and make a BOM in order to get a budgetory estimate.
You can have a look to the most common distributors, including LCSC that has the lowest prices.
If you want to lower the price, try to select part-numbers from their catalogue