I want to place an ad to sell my services. Should I say I have good service at a low price or low cost?
Price is what you charge; cost is what the buyer winds up paying, which may be more or less than the price, and in any event is not under your control.
See this paper for entirely too many details about these words, as well as worth and value.
Definitely Low Price over Low Cost.
Low cost implies low quality or the low end solution. Low price implies a bargain.
People get a lot more excited about bargains than products that are cheap because they don't cost much. A low cost good is something that has little value. A low priced good may actually cost less than it's value.