You will always be asked to give recommendations or questions on how much a part or even a computer costs.  One of the jobs of a technician is to keep up with the current market prices.  The following are some websites that you can use to find out prices for new or used computer related items.

http://www.pricewatch.com – Compares prices of websites all over the internet.  Organized by device type.

http://www.froogle.com – Similar to pricewatch.com, but uses Google’s search engines

http://www.ebay.com – Great source for looking up prices of older products

http://www.tigerdirect.com – One of the leading retail stores on the net.

http://www.cdw.com - Another retail store.

http://www.newegg.com - A retail store for the true geek.

Direct Google Search - just search for the make and model directly on http://www.google.com.