Unfortunately, this is probably not illegal in any way, as a price list would not generally be considered a trade secret.
Here is an article on the issue: http://www.zdnet.com/blog/btl/technology-pricing-follies-emc-claims-price-list-is-a-trade-secret/4696
Unless your price list contained special concessions, etc., or your client signed a confidentiality agreement regarding the price list, it is unlikely you would be able to recover. However, you could try writing a cease and desist letter as mentioned in the article, but this would probably end your relationship with the client, from a business perspective.
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