PCI as in the second form of "expansion" slot that was added to the industry list of standards after ISA, is the CHEAPEST. But as far as graphics cards go, they don't make PCI based graphics cards anymore. You'd either have to have one from an old system or one that you bought a few years ago, or try and find one on Ebay or something.
As for the current standards, AGP and PCI-E (aka PCI-Express). AGP is cheaper because it has been out longer, and has already reached what designers have believed to be it's peak of performance capabilities. PCI-E which has only been out within the past couple years has brought about new possibilities but unfortunately at the moment, you do pay more for it.
The only unfortunate thing the new PCI-E era has brought about, is that for users to upgrade to PCI-E graphics cards they have to replace their motherboards, which usually means replacing the CPU and memory as well. Which while to take full advantage of the new technologies, it isn't a bad thing...the downside of course becomes the cost.
So in the end, the while overall PCI-E is the best option as it is where the future is heading, at least until something new comes around, whereas AGP is becoming a fading star; many users decisions will still be based upon their pocketbooks.