価格比較サイトって英語でなんて言うの？

More briefly, you may just call it 'a comparison site.'
Obviously some sites do not compare price but may compare other factors such as location, service, satisfaction levels, scores of the customers for this service or item.
Especially in terms of insurance for cars or homes, buying electrical products for the home or mobile phones - there is no end of comparison sites on the net.
Price is one of the most important factors obviously and will be included on practically every comparison site.

When you want to talk about a website that you can use to compare prices for products; then you can use the following phrases:
-A website to compare product prices
-A price comparison website

In a sentence, you could perhaps say:
-I visited a few price comparison websites so that I can see which hotel is cheaper.
-I want to build my own website to compare product prices in Japan.

this compares prices from many websites

In South Africa we have a website that compares prices for a product for you within South Africa and you can choose the cheapest product for you. Other countries use the same website but with different name.

These are all ways to describe these type of websites.

"Website" can be shorted to the word "site"

To 'compare' something or to make a 'comparison' means to tell the difference between 2 or more things. There are a lot of different comparison websites especially for insurance (Gocompare/Compare the Market) these do not only compare the price but other things as well

When you are looking to purchase something you naturally would like the best deal and lowest price. There are particular websites you can visit called price comparison websites that make this job easier for you. You can type in the keyword and the price comparison website will show you many different outlets for the same or similar products at different prices so you can asses the merchandise and decide which one is offering the best product for the best price.