Use of Open Source Products

I’m new to the use of Open Source products and hence my view may not be correct, I feel that Open Source Products may decrease the cost of software development, but the cost of maintenance of such products will be very high and since the major cost of a software comes in maintaining, the over all cost will even out to as much as it used to be in the past ie with stable established products.
Please give your views in this matter and whether my view is valid or not.