You need to look closely at the reason the plug fouled.If it was a coil pack you would definately have a check engine light on.Either a code P030x or P035x,x being the # of the cyl.If the coil has been compromised,ie,cracked and moisture corroded the wiring,just replace the one coil pack.The final decision is yours,COP's go anywhere from $15 on E-bay to about $100 from the dealer.If you have a CEL on,go to a parts store most have scan tools that they will read the code for you.let us know what you find.
Pat
Pat