I don't think that there is anything illegal in doing so. However you can take few steps for precautions. The web page where you show this comparision, also show a disclaimer like
"These results are obtained from respective companies web sites, and are subject to change without notice."
You can also put the date when you got those results. But if you are maintaining more than one web site, I would say, skip the part of putting the date.
Mention the company's name clearly without tempering with their Title. If you wish, you can also link the title to that company's home page.
Well if you do so, you are good to go. Legally they cannot do anything to you. And even if they don't like it, if you keep update with their price they cannot do anything. Cuz all what you are doing is providing consumers the necessary "Information".
Anybody please correct me if I am wrong.
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