In the daytime, the attic space just above your living areas accumulates heat and while insulation does provide a barrier to stop migration of heat into the living spaces, it does not always work well enough. Hence, if your night is 70 degrees outside, yet, inside it is 80 degrees, you may opt to turn on the AC to cool down the inside and waste electricity.
It IS a common issue. Some homes built have insufficient rood vents to remove the hot air and if there is too much of it, it will invade the living areas. So, what are the formulas to determine your needs?
1. To determine if you need more roof vents, use the standard 300 sq.ft. rule, which states for every 300 sf, you need one roof vent. So, divide the square footage of your home by 300. For example, a 2000 sf home would need at least 7 roof vents (2000 divided by 300 = about 7). If you look at your roof vents and have less that that amount, you should add some to drop the attic temperature.
2. Once you determine you need more roof vents, you need to the CFM (Cubic Feet per Minute) needed for a room space that a vent needs to remove air from per hour. For a general idea, measure the room width and length (12x12ft=144 sq. ft) then multiply 144 sf x 0.7 = 100 cfm. The 100 cfm is the minimum needed to have air low exchanged per hour. Use this information when looking for a roof vent in their specifications as you want one with at least 100 cfm.
As for having them installed, unless your doing it yourself, leave the placement to the contractor, but as a rule, the cost is around $300 per vent or more depending on the cost of the vent itself that can range from $50 to $400. It takes about 2 hrs. to install one vent.
Usually, the vents with solar fans to draw the hot air from the attic are between $200-400. While they work well to reduce the heat, should they fail, then it becomes an expensive passive air vent if not fixed, and they do fail. If you have good air flow, passive vents are the way to go.