Member Rara Avis
All three of these are bus interfaces - or ways for a card to talk to the system. Think of a bus as a set of parallel conductors, which allow devices attached to it (the cards) to communicate with the CPU. The goal of a good bus is to run at about the same speed as the computer. As computers have gotten faster and faster, we've had to come up with new bus architectures.
AGP stands for Accelerated Graphics Port, and that's exactly what it is. It enables high performance graphics, especially 3D, and you'd probably be hard pressed to find any PC video card not using it today. That standard before AGP was called VESA, and before that we were using plain old ISA video cards.
ISA stands for Industry Standard Architecture, the "old" way that goes back to about 1981. It operates at only 8 MHz, which today seems really really slow. It was, for a brief time, resurrected as EISA (Enhanced ISA), which was much faster - but still slower than even a 33 MHz CPU. IBM tried to use an entirely different bus system, about the same time EISA was popular, but it never became popular outside their own machines. I'm stretching my memory cells here, but I think it was called MCI.
PCI stands for Peripheral Component Interconnect, and is the NEW way of doing things. The bus is 64 bits wide (as compared the original 16 bits of ISA) and runs at 66 MHz, 133 MHz, or (on the new P4 systems) a whopping 400 MHz. These faster speeds mean that our new computers don't have to sit around twiddling their thumbs while they wait for the interface cards to answer them. Were it not for the new bus standards (which really took forever to surface), today's computers would only be marginally faster than those of twenty years ago - because they'd spend all their time just waiting.
I had to really laugh when you said ISA cost more than PCI. I can assure you, it sure wasn't that way when PCI first came off the assembly line. I can only guess the turn-around is due to economy of scale. When you only make 442 of something (oops, guess I've been thinking of Voices too much lately), each one is going to cost a lot. But when you make 40 million of something, each one costs a lot less. I can only assume they're not making a lot of ISA boards these days, so the cost has risen accordingly.