My 89 doesn't run and that's how i bought it. I don't mind it really, but i need some advice. All I know is that I blew the main fuse, and I'm wondering how much it would cost to replace it.

 

A new main fuse will likely cost next to nothing. Try an auto parts store. But in order to blow the main fuse, something must have shorted out or the battery was put in backwards. If it's the second option, a number of circuit boards will have fried.

 

The main fuse is usually bolted on the engine bay fuse block.On each side you will find a 10 mm bolt that holds the fuse IN to the Fuse Tray(block) so it doesn't accidentally wiggle out and cause the car to stop running.
RR88 is right that the fuse is only about 6-8 dollars from a store Like NAPA.
Try to Correctly Identify the wires that attach to the battery,and as a "rule of Thumb" what I do is get some RED and Black electrical tape(dollar store) and just wind it a coupls wraps around the Positive(RED) cable and negative(Black) cable.
the S4 I know have a NEGative cable with a Yellow stripe on it for the Battery's Negative cable Connector..(it is a little reverse you would think,but that is how they mark it!)
and ya..putting the cables on in reverse will Blow the MAIN fuse and IF you are really unfortunate,it may take the ECU with it,and perhaps a Headlight switch(I think).
Good luck to ya.STYX.

 

Check for corsion while your in that area I had a weird problem that every time I turned left the car would cut out... it was caused of corrsion there..

 

Its just a 100amp fuse from the parts store. Couple bucks. Make sure your fix the short that blew the main fuse in the first place, or else it will just blow again.