Cheap is generally a relative term, a particular amount, say$50 can be cheap for some and expensive for some. Hence the tariffs that claim to be cheap are also relative to people. Hence the best thing that would be unanimously cheap for all would be free. There are various ways of charging for a call that might suit people. There are flat charges or bulk charges. Flat charging is basically for those who happen to be regular users of the service. For reduced rates the service provider gives a secret pass code on which you all can come and chat over. They are charged the minimum cents per second, totally as per the plan they take up. But if you happen to be very regular customer then he might charge you a flat rate, which means irrespective of the minutes you talk a particular month you pay a fixed amount of money.This way you can make as many calls you need and save the money; for the more calls you make the more money you save. This is quite near to the free conference calling. These services are generally provided by those who own the tele-conferencing services and do not have to outsource for it.
But paying no money is the best plan. Hence you can check out the free conference calling plans. Most of times these call plans are for advertising purposes. After a particular amount of time you need to buy a paid plan. But yes, if you use computer to make these conference calls you would get them for free. The connectivity depends on your internet connection, but the clarity and the audio quality is best in the market. This service can also be taken into for business purposes.