With summer nearing, I'm considering options. I have multiple turbine vents, but we store quite a bit in the two attic areas we have, and I'd like to find the most cost-friendly adjustment to lower temperatures.

I don't know if this qualifies as "cheapest" but my AC guy recommended Attic bBreeze fans to bring attic temperatures down.

We haven't gotten around to installing one yet, but we also store nothing in our attic.

http://www.atticbreeze.net

Call your SOP roofer
He will cut/install ridge vents
and supply soffit vents if needed
while insuring ventilation calculations are to CODE

If your "designer" did a poor roof plan = only turbines (soooo sorry for these poor vents)
then might pony up the $$$$$ for foamed attic

As stated, there are plenty of ways to skin this cat! Not all are cheap. The first thing you need to do is utilize the Owens Corning ventilation calculator and figure out how much ventilation you need. Many houses don't have enough ridge vent to properly ventilate the roof and attic. In those cases, turtle vents, turbines, etc are utilized. Personally, if I need to ventilate an attic, it go with a solar vent like attic breeze or Kennedy. They have long warranties, thermostats, and require no wiring which means your roofer can install it start to finish. I also prefer them because I can use two solar vents where I might need 15 turtle vents. They are not cheap, but they are worth it. Quality and efficiency isn't cheap.

Thank you guys!

Blow in more insulation. You might check with your power company, some offer rebates.

Attic foil is your easiest and cheapest solution that only requires scissors and a staple gun to install.

https://www.atticfoil.com