Joe-Short of "shotgunning" (changing) parts randomly, which will be quite expensive as you've found out, you need a plan. Troubleshooting electrical, it is best to start at one end, usually the end that isn't working (spark plug), and work your way upstream until you find where it is working. Your problem will be there. You are going to have to have a manual with a schematic, and a good working knowledge of how things flow to the spark plug. Truth is, without seeing the bike in person, it's difficult at best for anyone here to things other than offer suggestions about what it could be. It could be any one of a hundred things causing a lack of spark. Like I said, short of just jumping around all over the ignition system looking for problems until you find it, you need to work methodically starting at the plug until you find the point where the signal is dropping.