Not to be flippant but what is your budget?
For the most part, RAM pricing is based on availability - market factors determine the prices.
Also make and model laptop? Current RAM configuration?
Start by visiting RAM manufacturer's websites (Crucial, Kingston, PNY, Samsung)
Identify the applicable RAM modules and configuration options for your laptop. Be very careful - details matter.
Then price (via the manufacturer's website) the necessary supported RAM modules. Most likely you will need a matched set (dual channel).
Then go online and look for the required/desired RAM modules. Price per online vendor (e.g., Amazon) and do the same for reseller sites (e.g., E-bay).
Keep notes regarding RAM options and the corresponding prices.
Then you will be able to decide what and how to purchase. Be wary of used RAM and ensure that you can return a product that does not work. (RMA and refund.)