MIDI stands for musical instrument digital interface. basicly it is a way of digitally reproducing sounds that come from instruments. its a digital representation of sound, as opposed to analog.

VST stands for virtual studio technology. basicly VSTs are plugins that you can use with various music programs that allow you to use the sounds of an instrument, create sounds, use various effects, or similar things.

since neither is one actual item, i cant give you a cost.
i thought midi was something that i could plug my guitar into that goes into my computer to rec the guitar
what programs do you have? you can do quite a bit with midi, so let me know what programs you are using and ill try to help you out a bit.

and please dont double post. instead use the edit button on the bottom right hand corner of your post.